+++ /dev/null
-Crossrelease
-============
-
-Started by Byungchul Park <byungchul.park@lge.com>
-
-Contents:
-
- (*) Background
-
- - What causes deadlock
- - How lockdep works
-
- (*) Limitation
-
- - Limit lockdep
- - Pros from the limitation
- - Cons from the limitation
- - Relax the limitation
-
- (*) Crossrelease
-
- - Introduce crossrelease
- - Introduce commit
-
- (*) Implementation
-
- - Data structures
- - How crossrelease works
-
- (*) Optimizations
-
- - Avoid duplication
- - Lockless for hot paths
-
- (*) APPENDIX A: What lockdep does to work aggresively
-
- (*) APPENDIX B: How to avoid adding false dependencies
-
-
-==========
-Background
-==========
-
-What causes deadlock
---------------------
-
-A deadlock occurs when a context is waiting for an event to happen,
-which is impossible because another (or the) context who can trigger the
-event is also waiting for another (or the) event to happen, which is
-also impossible due to the same reason.
-
-For example:
-
- A context going to trigger event C is waiting for event A to happen.
- A context going to trigger event A is waiting for event B to happen.
- A context going to trigger event B is waiting for event C to happen.
-
-A deadlock occurs when these three wait operations run at the same time,
-because event C cannot be triggered if event A does not happen, which in
-turn cannot be triggered if event B does not happen, which in turn
-cannot be triggered if event C does not happen. After all, no event can
-be triggered since any of them never meets its condition to wake up.
-
-A dependency might exist between two waiters and a deadlock might happen
-due to an incorrect releationship between dependencies. Thus, we must
-define what a dependency is first. A dependency exists between them if:
-
- 1. There are two waiters waiting for each event at a given time.
- 2. The only way to wake up each waiter is to trigger its event.
- 3. Whether one can be woken up depends on whether the other can.
-
-Each wait in the example creates its dependency like:
-
- Event C depends on event A.
- Event A depends on event B.
- Event B depends on event C.
-
- NOTE: Precisely speaking, a dependency is one between whether a
- waiter for an event can be woken up and whether another waiter for
- another event can be woken up. However from now on, we will describe
- a dependency as if it's one between an event and another event for
- simplicity.
-
-And they form circular dependencies like:
-
- -> C -> A -> B -
- / \
- \ /
- ----------------
-
- where 'A -> B' means that event A depends on event B.
-
-Such circular dependencies lead to a deadlock since no waiter can meet
-its condition to wake up as described.
-
-CONCLUSION
-
-Circular dependencies cause a deadlock.
-
-
-How lockdep works
------------------
-
-Lockdep tries to detect a deadlock by checking dependencies created by
-lock operations, acquire and release. Waiting for a lock corresponds to
-waiting for an event, and releasing a lock corresponds to triggering an
-event in the previous section.
-
-In short, lockdep does:
-
- 1. Detect a new dependency.
- 2. Add the dependency into a global graph.
- 3. Check if that makes dependencies circular.
- 4. Report a deadlock or its possibility if so.
-
-For example, consider a graph built by lockdep that looks like:
-
- A -> B -
- \
- -> E
- /
- C -> D -
-
- where A, B,..., E are different lock classes.
-
-Lockdep will add a dependency into the graph on detection of a new
-dependency. For example, it will add a dependency 'E -> C' when a new
-dependency between lock E and lock C is detected. Then the graph will be:
-
- A -> B -
- \
- -> E -
- / \
- -> C -> D - \
- / /
- \ /
- ------------------
-
- where A, B,..., E are different lock classes.
-
-This graph contains a subgraph which demonstrates circular dependencies:
-
- -> E -
- / \
- -> C -> D - \
- / /
- \ /
- ------------------
-
- where C, D and E are different lock classes.
-
-This is the condition under which a deadlock might occur. Lockdep
-reports it on detection after adding a new dependency. This is the way
-how lockdep works.
-
-CONCLUSION
-
-Lockdep detects a deadlock or its possibility by checking if circular
-dependencies were created after adding each new dependency.
-
-
-==========
-Limitation
-==========
-
-Limit lockdep
--------------
-
-Limiting lockdep to work on only typical locks e.g. spin locks and
-mutexes, which are released within the acquire context, the
-implementation becomes simple but its capacity for detection becomes
-limited. Let's check pros and cons in next section.
-
-
-Pros from the limitation
-------------------------
-
-Given the limitation, when acquiring a lock, locks in a held_locks
-cannot be released if the context cannot acquire it so has to wait to
-acquire it, which means all waiters for the locks in the held_locks are
-stuck. It's an exact case to create dependencies between each lock in
-the held_locks and the lock to acquire.
-
-For example:
-
- CONTEXT X
- ---------
- acquire A
- acquire B /* Add a dependency 'A -> B' */
- release B
- release A
-
- where A and B are different lock classes.
-
-When acquiring lock A, the held_locks of CONTEXT X is empty thus no
-dependency is added. But when acquiring lock B, lockdep detects and adds
-a new dependency 'A -> B' between lock A in the held_locks and lock B.
-They can be simply added whenever acquiring each lock.
-
-And data required by lockdep exists in a local structure, held_locks
-embedded in task_struct. Forcing to access the data within the context,
-lockdep can avoid racy problems without explicit locks while handling
-the local data.
-
-Lastly, lockdep only needs to keep locks currently being held, to build
-a dependency graph. However, relaxing the limitation, it needs to keep
-even locks already released, because a decision whether they created
-dependencies might be long-deferred.
-
-To sum up, we can expect several advantages from the limitation:
-
- 1. Lockdep can easily identify a dependency when acquiring a lock.
- 2. Races are avoidable while accessing local locks in a held_locks.
- 3. Lockdep only needs to keep locks currently being held.
-
-CONCLUSION
-
-Given the limitation, the implementation becomes simple and efficient.
-
-
-Cons from the limitation
-------------------------
-
-Given the limitation, lockdep is applicable only to typical locks. For
-example, page locks for page access or completions for synchronization
-cannot work with lockdep.
-
-Can we detect deadlocks below, under the limitation?
-
-Example 1:
-
- CONTEXT X CONTEXT Y CONTEXT Z
- --------- --------- ----------
- mutex_lock A
- lock_page B
- lock_page B
- mutex_lock A /* DEADLOCK */
- unlock_page B held by X
- unlock_page B
- mutex_unlock A
- mutex_unlock A
-
- where A and B are different lock classes.
-
-No, we cannot.
-
-Example 2:
-
- CONTEXT X CONTEXT Y
- --------- ---------
- mutex_lock A
- mutex_lock A
- wait_for_complete B /* DEADLOCK */
- complete B
- mutex_unlock A
- mutex_unlock A
-
- where A is a lock class and B is a completion variable.
-
-No, we cannot.
-
-CONCLUSION
-
-Given the limitation, lockdep cannot detect a deadlock or its
-possibility caused by page locks or completions.
-
-
-Relax the limitation
---------------------
-
-Under the limitation, things to create dependencies are limited to
-typical locks. However, synchronization primitives like page locks and
-completions, which are allowed to be released in any context, also
-create dependencies and can cause a deadlock. So lockdep should track
-these locks to do a better job. We have to relax the limitation for
-these locks to work with lockdep.
-
-Detecting dependencies is very important for lockdep to work because
-adding a dependency means adding an opportunity to check whether it
-causes a deadlock. The more lockdep adds dependencies, the more it
-thoroughly works. Thus Lockdep has to do its best to detect and add as
-many true dependencies into a graph as possible.
-
-For example, considering only typical locks, lockdep builds a graph like:
-
- A -> B -
- \
- -> E
- /
- C -> D -
-
- where A, B,..., E are different lock classes.
-
-On the other hand, under the relaxation, additional dependencies might
-be created and added. Assuming additional 'FX -> C' and 'E -> GX' are
-added thanks to the relaxation, the graph will be:
-
- A -> B -
- \
- -> E -> GX
- /
- FX -> C -> D -
-
- where A, B,..., E, FX and GX are different lock classes, and a suffix
- 'X' is added on non-typical locks.
-
-The latter graph gives us more chances to check circular dependencies
-than the former. However, it might suffer performance degradation since
-relaxing the limitation, with which design and implementation of lockdep
-can be efficient, might introduce inefficiency inevitably. So lockdep
-should provide two options, strong detection and efficient detection.
-
-Choosing efficient detection:
-
- Lockdep works with only locks restricted to be released within the
- acquire context. However, lockdep works efficiently.
-
-Choosing strong detection:
-
- Lockdep works with all synchronization primitives. However, lockdep
- suffers performance degradation.
-
-CONCLUSION
-
-Relaxing the limitation, lockdep can add additional dependencies giving
-additional opportunities to check circular dependencies.
-
-
-============
-Crossrelease
-============
-
-Introduce crossrelease
-----------------------
-
-In order to allow lockdep to handle additional dependencies by what
-might be released in any context, namely 'crosslock', we have to be able
-to identify those created by crosslocks. The proposed 'crossrelease'
-feature provoides a way to do that.
-
-Crossrelease feature has to do:
-
- 1. Identify dependencies created by crosslocks.
- 2. Add the dependencies into a dependency graph.
-
-That's all. Once a meaningful dependency is added into graph, then
-lockdep would work with the graph as it did. The most important thing
-crossrelease feature has to do is to correctly identify and add true
-dependencies into the global graph.
-
-A dependency e.g. 'A -> B' can be identified only in the A's release
-context because a decision required to identify the dependency can be
-made only in the release context. That is to decide whether A can be
-released so that a waiter for A can be woken up. It cannot be made in
-other than the A's release context.
-
-It's no matter for typical locks because each acquire context is same as
-its release context, thus lockdep can decide whether a lock can be
-released in the acquire context. However for crosslocks, lockdep cannot
-make the decision in the acquire context but has to wait until the
-release context is identified.
-
-Therefore, deadlocks by crosslocks cannot be detected just when it
-happens, because those cannot be identified until the crosslocks are
-released. However, deadlock possibilities can be detected and it's very
-worth. See 'APPENDIX A' section to check why.
-
-CONCLUSION
-
-Using crossrelease feature, lockdep can work with what might be released
-in any context, namely crosslock.
-
-
-Introduce commit
-----------------
-
-Since crossrelease defers the work adding true dependencies of
-crosslocks until they are actually released, crossrelease has to queue
-all acquisitions which might create dependencies with the crosslocks.
-Then it identifies dependencies using the queued data in batches at a
-proper time. We call it 'commit'.
-
-There are four types of dependencies:
-
-1. TT type: 'typical lock A -> typical lock B'
-
- Just when acquiring B, lockdep can see it's in the A's release
- context. So the dependency between A and B can be identified
- immediately. Commit is unnecessary.
-
-2. TC type: 'typical lock A -> crosslock BX'
-
- Just when acquiring BX, lockdep can see it's in the A's release
- context. So the dependency between A and BX can be identified
- immediately. Commit is unnecessary, too.
-
-3. CT type: 'crosslock AX -> typical lock B'
-
- When acquiring B, lockdep cannot identify the dependency because
- there's no way to know if it's in the AX's release context. It has
- to wait until the decision can be made. Commit is necessary.
-
-4. CC type: 'crosslock AX -> crosslock BX'
-
- When acquiring BX, lockdep cannot identify the dependency because
- there's no way to know if it's in the AX's release context. It has
- to wait until the decision can be made. Commit is necessary.
- But, handling CC type is not implemented yet. It's a future work.
-
-Lockdep can work without commit for typical locks, but commit step is
-necessary once crosslocks are involved. Introducing commit, lockdep
-performs three steps. What lockdep does in each step is:
-
-1. Acquisition: For typical locks, lockdep does what it originally did
- and queues the lock so that CT type dependencies can be checked using
- it at the commit step. For crosslocks, it saves data which will be
- used at the commit step and increases a reference count for it.
-
-2. Commit: No action is reauired for typical locks. For crosslocks,
- lockdep adds CT type dependencies using the data saved at the
- acquisition step.
-
-3. Release: No changes are required for typical locks. When a crosslock
- is released, it decreases a reference count for it.
-
-CONCLUSION
-
-Crossrelease introduces commit step to handle dependencies of crosslocks
-in batches at a proper time.
-
-
-==============
-Implementation
-==============
-
-Data structures
----------------
-
-Crossrelease introduces two main data structures.
-
-1. hist_lock
-
- This is an array embedded in task_struct, for keeping lock history so
- that dependencies can be added using them at the commit step. Since
- it's local data, it can be accessed locklessly in the owner context.
- The array is filled at the acquisition step and consumed at the
- commit step. And it's managed in circular manner.
-
-2. cross_lock
-
- One per lockdep_map exists. This is for keeping data of crosslocks
- and used at the commit step.
-
-
-How crossrelease works
-----------------------
-
-It's the key of how crossrelease works, to defer necessary works to an
-appropriate point in time and perform in at once at the commit step.
-Let's take a look with examples step by step, starting from how lockdep
-works without crossrelease for typical locks.
-
- acquire A /* Push A onto held_locks */
- acquire B /* Push B onto held_locks and add 'A -> B' */
- acquire C /* Push C onto held_locks and add 'B -> C' */
- release C /* Pop C from held_locks */
- release B /* Pop B from held_locks */
- release A /* Pop A from held_locks */
-
- where A, B and C are different lock classes.
-
- NOTE: This document assumes that readers already understand how
- lockdep works without crossrelease thus omits details. But there's
- one thing to note. Lockdep pretends to pop a lock from held_locks
- when releasing it. But it's subtly different from the original pop
- operation because lockdep allows other than the top to be poped.
-
-In this case, lockdep adds 'the top of held_locks -> the lock to acquire'
-dependency every time acquiring a lock.
-
-After adding 'A -> B', a dependency graph will be:
-
- A -> B
-
- where A and B are different lock classes.
-
-And after adding 'B -> C', the graph will be:
-
- A -> B -> C
-
- where A, B and C are different lock classes.
-
-Let's performs commit step even for typical locks to add dependencies.
-Of course, commit step is not necessary for them, however, it would work
-well because this is a more general way.
-
- acquire A
- /*
- * Queue A into hist_locks
- *
- * In hist_locks: A
- * In graph: Empty
- */
-
- acquire B
- /*
- * Queue B into hist_locks
- *
- * In hist_locks: A, B
- * In graph: Empty
- */
-
- acquire C
- /*
- * Queue C into hist_locks
- *
- * In hist_locks: A, B, C
- * In graph: Empty
- */
-
- commit C
- /*
- * Add 'C -> ?'
- * Answer the following to decide '?'
- * What has been queued since acquire C: Nothing
- *
- * In hist_locks: A, B, C
- * In graph: Empty
- */
-
- release C
-
- commit B
- /*
- * Add 'B -> ?'
- * Answer the following to decide '?'
- * What has been queued since acquire B: C
- *
- * In hist_locks: A, B, C
- * In graph: 'B -> C'
- */
-
- release B
-
- commit A
- /*
- * Add 'A -> ?'
- * Answer the following to decide '?'
- * What has been queued since acquire A: B, C
- *
- * In hist_locks: A, B, C
- * In graph: 'B -> C', 'A -> B', 'A -> C'
- */
-
- release A
-
- where A, B and C are different lock classes.
-
-In this case, dependencies are added at the commit step as described.
-
-After commits for A, B and C, the graph will be:
-
- A -> B -> C
-
- where A, B and C are different lock classes.
-
- NOTE: A dependency 'A -> C' is optimized out.
-
-We can see the former graph built without commit step is same as the
-latter graph built using commit steps. Of course the former way leads to
-earlier finish for building the graph, which means we can detect a
-deadlock or its possibility sooner. So the former way would be prefered
-when possible. But we cannot avoid using the latter way for crosslocks.
-
-Let's look at how commit steps work for crosslocks. In this case, the
-commit step is performed only on crosslock AX as real. And it assumes
-that the AX release context is different from the AX acquire context.
-
- BX RELEASE CONTEXT BX ACQUIRE CONTEXT
- ------------------ ------------------
- acquire A
- /*
- * Push A onto held_locks
- * Queue A into hist_locks
- *
- * In held_locks: A
- * In hist_locks: A
- * In graph: Empty
- */
-
- acquire BX
- /*
- * Add 'the top of held_locks -> BX'
- *
- * In held_locks: A
- * In hist_locks: A
- * In graph: 'A -> BX'
- */
-
- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
- It must be guaranteed that the following operations are seen after
- acquiring BX globally. It can be done by things like barrier.
- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-
- acquire C
- /*
- * Push C onto held_locks
- * Queue C into hist_locks
- *
- * In held_locks: C
- * In hist_locks: C
- * In graph: 'A -> BX'
- */
-
- release C
- /*
- * Pop C from held_locks
- *
- * In held_locks: Empty
- * In hist_locks: C
- * In graph: 'A -> BX'
- */
- acquire D
- /*
- * Push D onto held_locks
- * Queue D into hist_locks
- * Add 'the top of held_locks -> D'
- *
- * In held_locks: A, D
- * In hist_locks: A, D
- * In graph: 'A -> BX', 'A -> D'
- */
- acquire E
- /*
- * Push E onto held_locks
- * Queue E into hist_locks
- *
- * In held_locks: E
- * In hist_locks: C, E
- * In graph: 'A -> BX', 'A -> D'
- */
-
- release E
- /*
- * Pop E from held_locks
- *
- * In held_locks: Empty
- * In hist_locks: D, E
- * In graph: 'A -> BX', 'A -> D'
- */
- release D
- /*
- * Pop D from held_locks
- *
- * In held_locks: A
- * In hist_locks: A, D
- * In graph: 'A -> BX', 'A -> D'
- */
- commit BX
- /*
- * Add 'BX -> ?'
- * What has been queued since acquire BX: C, E
- *
- * In held_locks: Empty
- * In hist_locks: D, E
- * In graph: 'A -> BX', 'A -> D',
- * 'BX -> C', 'BX -> E'
- */
-
- release BX
- /*
- * In held_locks: Empty
- * In hist_locks: D, E
- * In graph: 'A -> BX', 'A -> D',
- * 'BX -> C', 'BX -> E'
- */
- release A
- /*
- * Pop A from held_locks
- *
- * In held_locks: Empty
- * In hist_locks: A, D
- * In graph: 'A -> BX', 'A -> D',
- * 'BX -> C', 'BX -> E'
- */
-
- where A, BX, C,..., E are different lock classes, and a suffix 'X' is
- added on crosslocks.
-
-Crossrelease considers all acquisitions after acqiuring BX are
-candidates which might create dependencies with BX. True dependencies
-will be determined when identifying the release context of BX. Meanwhile,
-all typical locks are queued so that they can be used at the commit step.
-And then two dependencies 'BX -> C' and 'BX -> E' are added at the
-commit step when identifying the release context.
-
-The final graph will be, with crossrelease:
-
- -> C
- /
- -> BX -
- / \
- A - -> E
- \
- -> D
-
- where A, BX, C,..., E are different lock classes, and a suffix 'X' is
- added on crosslocks.
-
-However, the final graph will be, without crossrelease:
-
- A -> D
-
- where A and D are different lock classes.
-
-The former graph has three more dependencies, 'A -> BX', 'BX -> C' and
-'BX -> E' giving additional opportunities to check if they cause
-deadlocks. This way lockdep can detect a deadlock or its possibility
-caused by crosslocks.
-
-CONCLUSION
-
-We checked how crossrelease works with several examples.
-
-
-=============
-Optimizations
-=============
-
-Avoid duplication
------------------
-
-Crossrelease feature uses a cache like what lockdep already uses for
-dependency chains, but this time it's for caching CT type dependencies.
-Once that dependency is cached, the same will never be added again.
-
-
-Lockless for hot paths
-----------------------
-
-To keep all locks for later use at the commit step, crossrelease adopts
-a local array embedded in task_struct, which makes access to the data
-lockless by forcing it to happen only within the owner context. It's
-like how lockdep handles held_locks. Lockless implmentation is important
-since typical locks are very frequently acquired and released.
-
-
-=================================================
-APPENDIX A: What lockdep does to work aggresively
-=================================================
-
-A deadlock actually occurs when all wait operations creating circular
-dependencies run at the same time. Even though they don't, a potential
-deadlock exists if the problematic dependencies exist. Thus it's
-meaningful to detect not only an actual deadlock but also its potential
-possibility. The latter is rather valuable. When a deadlock occurs
-actually, we can identify what happens in the system by some means or
-other even without lockdep. However, there's no way to detect possiblity
-without lockdep unless the whole code is parsed in head. It's terrible.
-Lockdep does the both, and crossrelease only focuses on the latter.
-
-Whether or not a deadlock actually occurs depends on several factors.
-For example, what order contexts are switched in is a factor. Assuming
-circular dependencies exist, a deadlock would occur when contexts are
-switched so that all wait operations creating the dependencies run
-simultaneously. Thus to detect a deadlock possibility even in the case
-that it has not occured yet, lockdep should consider all possible
-combinations of dependencies, trying to:
-
-1. Use a global dependency graph.
-
- Lockdep combines all dependencies into one global graph and uses them,
- regardless of which context generates them or what order contexts are
- switched in. Aggregated dependencies are only considered so they are
- prone to be circular if a problem exists.
-
-2. Check dependencies between classes instead of instances.
-
- What actually causes a deadlock are instances of lock. However,
- lockdep checks dependencies between classes instead of instances.
- This way lockdep can detect a deadlock which has not happened but
- might happen in future by others but the same class.
-
-3. Assume all acquisitions lead to waiting.
-
- Although locks might be acquired without waiting which is essential
- to create dependencies, lockdep assumes all acquisitions lead to
- waiting since it might be true some time or another.
-
-CONCLUSION
-
-Lockdep detects not only an actual deadlock but also its possibility,
-and the latter is more valuable.
-
-
-==================================================
-APPENDIX B: How to avoid adding false dependencies
-==================================================
-
-Remind what a dependency is. A dependency exists if:
-
- 1. There are two waiters waiting for each event at a given time.
- 2. The only way to wake up each waiter is to trigger its event.
- 3. Whether one can be woken up depends on whether the other can.
-
-For example:
-
- acquire A
- acquire B /* A dependency 'A -> B' exists */
- release B
- release A
-
- where A and B are different lock classes.
-
-A depedency 'A -> B' exists since:
-
- 1. A waiter for A and a waiter for B might exist when acquiring B.
- 2. Only way to wake up each is to release what it waits for.
- 3. Whether the waiter for A can be woken up depends on whether the
- other can. IOW, TASK X cannot release A if it fails to acquire B.
-
-For another example:
-
- TASK X TASK Y
- ------ ------
- acquire AX
- acquire B /* A dependency 'AX -> B' exists */
- release B
- release AX held by Y
-
- where AX and B are different lock classes, and a suffix 'X' is added
- on crosslocks.
-
-Even in this case involving crosslocks, the same rule can be applied. A
-depedency 'AX -> B' exists since:
-
- 1. A waiter for AX and a waiter for B might exist when acquiring B.
- 2. Only way to wake up each is to release what it waits for.
- 3. Whether the waiter for AX can be woken up depends on whether the
- other can. IOW, TASK X cannot release AX if it fails to acquire B.
-
-Let's take a look at more complicated example:
-
- TASK X TASK Y
- ------ ------
- acquire B
- release B
- fork Y
- acquire AX
- acquire C /* A dependency 'AX -> C' exists */
- release C
- release AX held by Y
-
- where AX, B and C are different lock classes, and a suffix 'X' is
- added on crosslocks.
-
-Does a dependency 'AX -> B' exist? Nope.
-
-Two waiters are essential to create a dependency. However, waiters for
-AX and B to create 'AX -> B' cannot exist at the same time in this
-example. Thus the dependency 'AX -> B' cannot be created.
-
-It would be ideal if the full set of true ones can be considered. But
-we can ensure nothing but what actually happened. Relying on what
-actually happens at runtime, we can anyway add only true ones, though
-they might be a subset of true ones. It's similar to how lockdep works
-for typical locks. There might be more true dependencies than what
-lockdep has detected in runtime. Lockdep has no choice but to rely on
-what actually happens. Crossrelease also relies on it.
-
-CONCLUSION
-
-Relying on what actually happens, lockdep can avoid adding false
-dependencies.
/*
* Prevent the compiler from merging or refetching reads or writes. The
* compiler is also forbidden from reordering successive instances of
- * READ_ONCE, WRITE_ONCE and ACCESS_ONCE (see below), but only when the
- * compiler is aware of some particular ordering. One way to make the
- * compiler aware of ordering is to put the two invocations of READ_ONCE,
- * WRITE_ONCE or ACCESS_ONCE() in different C statements.
+ * READ_ONCE and WRITE_ONCE, but only when the compiler is aware of some
+ * particular ordering. One way to make the compiler aware of ordering is to
+ * put the two invocations of READ_ONCE or WRITE_ONCE in different C
+ * statements.
*
- * In contrast to ACCESS_ONCE these two macros will also work on aggregate
- * data types like structs or unions. If the size of the accessed data
- * type exceeds the word size of the machine (e.g., 32 bits or 64 bits)
- * READ_ONCE() and WRITE_ONCE() will fall back to memcpy(). There's at
- * least two memcpy()s: one for the __builtin_memcpy() and then one for
- * the macro doing the copy of variable - '__u' allocated on the stack.
+ * These two macros will also work on aggregate data types like structs or
+ * unions. If the size of the accessed data type exceeds the word size of
+ * the machine (e.g., 32 bits or 64 bits) READ_ONCE() and WRITE_ONCE() will
+ * fall back to memcpy(). There's at least two memcpy()s: one for the
+ * __builtin_memcpy() and then one for the macro doing the copy of variable
+ * - '__u' allocated on the stack.
*
* Their two major use cases are: (1) Mediating communication between
* process-level code and irq/NMI handlers, all running on the same CPU,
- * and (2) Ensuring that the compiler does not fold, spindle, or otherwise
+ * and (2) Ensuring that the compiler does not fold, spindle, or otherwise
* mutilate accesses that either do not require ordering or that interact
* with an explicit memory barrier or atomic instruction that provides the
* required ordering.
compiletime_assert(__native_word(t), \
"Need native word sized stores/loads for atomicity.")
-/*
- * Prevent the compiler from merging or refetching accesses. The compiler
- * is also forbidden from reordering successive instances of ACCESS_ONCE(),
- * but only when the compiler is aware of some particular ordering. One way
- * to make the compiler aware of ordering is to put the two invocations of
- * ACCESS_ONCE() in different C statements.
- *
- * ACCESS_ONCE will only work on scalar types. For union types, ACCESS_ONCE
- * on a union member will work as long as the size of the member matches the
- * size of the union and the size is smaller than word size.
- *
- * The major use cases of ACCESS_ONCE used to be (1) Mediating communication
- * between process-level code and irq/NMI handlers, all running on the same CPU,
- * and (2) Ensuring that the compiler does not fold, spindle, or otherwise
- * mutilate accesses that either do not require ordering or that interact
- * with an explicit memory barrier or atomic instruction that provides the
- * required ordering.
- *
- * If possible use READ_ONCE()/WRITE_ONCE() instead.
- */
-#define __ACCESS_ONCE(x) ({ \
- __maybe_unused typeof(x) __var = (__force typeof(x)) 0; \
- (volatile typeof(x) *)&(x); })
-#define ACCESS_ONCE(x) (*__ACCESS_ONCE(x))
-
#endif /* __LINUX_COMPILER_H */
*/
#include <linux/wait.h>
-#ifdef CONFIG_LOCKDEP_COMPLETIONS
-#include <linux/lockdep.h>
-#endif
/*
* struct completion - structure used to maintain state for a "completion"
struct completion {
unsigned int done;
wait_queue_head_t wait;
-#ifdef CONFIG_LOCKDEP_COMPLETIONS
- struct lockdep_map_cross map;
-#endif
};
-#ifdef CONFIG_LOCKDEP_COMPLETIONS
-static inline void complete_acquire(struct completion *x)
-{
- lock_acquire_exclusive((struct lockdep_map *)&x->map, 0, 0, NULL, _RET_IP_);
-}
-
-static inline void complete_release(struct completion *x)
-{
- lock_release((struct lockdep_map *)&x->map, 0, _RET_IP_);
-}
-
-static inline void complete_release_commit(struct completion *x)
-{
- lock_commit_crosslock((struct lockdep_map *)&x->map);
-}
-
-#define init_completion_map(x, m) \
-do { \
- lockdep_init_map_crosslock((struct lockdep_map *)&(x)->map, \
- (m)->name, (m)->key, 0); \
- __init_completion(x); \
-} while (0)
-
-#define init_completion(x) \
-do { \
- static struct lock_class_key __key; \
- lockdep_init_map_crosslock((struct lockdep_map *)&(x)->map, \
- "(completion)" #x, \
- &__key, 0); \
- __init_completion(x); \
-} while (0)
-#else
#define init_completion_map(x, m) __init_completion(x)
#define init_completion(x) __init_completion(x)
static inline void complete_acquire(struct completion *x) {}
static inline void complete_release(struct completion *x) {}
static inline void complete_release_commit(struct completion *x) {}
-#endif
-#ifdef CONFIG_LOCKDEP_COMPLETIONS
-#define COMPLETION_INITIALIZER(work) \
- { 0, __WAIT_QUEUE_HEAD_INITIALIZER((work).wait), \
- STATIC_CROSS_LOCKDEP_MAP_INIT("(completion)" #work, &(work)) }
-#else
#define COMPLETION_INITIALIZER(work) \
{ 0, __WAIT_QUEUE_HEAD_INITIALIZER((work).wait) }
-#endif
#define COMPLETION_INITIALIZER_ONSTACK_MAP(work, map) \
(*({ init_completion_map(&(work), &(map)); &(work); }))
int cpu;
unsigned long ip;
#endif
-#ifdef CONFIG_LOCKDEP_CROSSRELEASE
- /*
- * Whether it's a crosslock.
- */
- int cross;
-#endif
};
static inline void lockdep_copy_map(struct lockdep_map *to,
unsigned int hardirqs_off:1;
unsigned int references:12; /* 32 bits */
unsigned int pin_count;
-#ifdef CONFIG_LOCKDEP_CROSSRELEASE
- /*
- * Generation id.
- *
- * A value of cross_gen_id will be stored when holding this,
- * which is globally increased whenever each crosslock is held.
- */
- unsigned int gen_id;
-#endif
-};
-
-#ifdef CONFIG_LOCKDEP_CROSSRELEASE
-#define MAX_XHLOCK_TRACE_ENTRIES 5
-
-/*
- * This is for keeping locks waiting for commit so that true dependencies
- * can be added at commit step.
- */
-struct hist_lock {
- /*
- * Id for each entry in the ring buffer. This is used to
- * decide whether the ring buffer was overwritten or not.
- *
- * For example,
- *
- * |<----------- hist_lock ring buffer size ------->|
- * pppppppppppppppppppppiiiiiiiiiiiiiiiiiiiiiiiiiiiii
- * wrapped > iiiiiiiiiiiiiiiiiiiiiiiiiii.......................
- *
- * where 'p' represents an acquisition in process
- * context, 'i' represents an acquisition in irq
- * context.
- *
- * In this example, the ring buffer was overwritten by
- * acquisitions in irq context, that should be detected on
- * rollback or commit.
- */
- unsigned int hist_id;
-
- /*
- * Seperate stack_trace data. This will be used at commit step.
- */
- struct stack_trace trace;
- unsigned long trace_entries[MAX_XHLOCK_TRACE_ENTRIES];
-
- /*
- * Seperate hlock instance. This will be used at commit step.
- *
- * TODO: Use a smaller data structure containing only necessary
- * data. However, we should make lockdep code able to handle the
- * smaller one first.
- */
- struct held_lock hlock;
};
-/*
- * To initialize a lock as crosslock, lockdep_init_map_crosslock() should
- * be called instead of lockdep_init_map().
- */
-struct cross_lock {
- /*
- * When more than one acquisition of crosslocks are overlapped,
- * we have to perform commit for them based on cross_gen_id of
- * the first acquisition, which allows us to add more true
- * dependencies.
- *
- * Moreover, when no acquisition of a crosslock is in progress,
- * we should not perform commit because the lock might not exist
- * any more, which might cause incorrect memory access. So we
- * have to track the number of acquisitions of a crosslock.
- */
- int nr_acquire;
-
- /*
- * Seperate hlock instance. This will be used at commit step.
- *
- * TODO: Use a smaller data structure containing only necessary
- * data. However, we should make lockdep code able to handle the
- * smaller one first.
- */
- struct held_lock hlock;
-};
-
-struct lockdep_map_cross {
- struct lockdep_map map;
- struct cross_lock xlock;
-};
-#endif
-
/*
* Initialization, self-test and debugging-output methods:
*/
XHLOCK_CTX_NR,
};
-#ifdef CONFIG_LOCKDEP_CROSSRELEASE
-extern void lockdep_init_map_crosslock(struct lockdep_map *lock,
- const char *name,
- struct lock_class_key *key,
- int subclass);
-extern void lock_commit_crosslock(struct lockdep_map *lock);
-
-/*
- * What we essencially have to initialize is 'nr_acquire'. Other members
- * will be initialized in add_xlock().
- */
-#define STATIC_CROSS_LOCK_INIT() \
- { .nr_acquire = 0,}
-
-#define STATIC_CROSS_LOCKDEP_MAP_INIT(_name, _key) \
- { .map.name = (_name), .map.key = (void *)(_key), \
- .map.cross = 1, .xlock = STATIC_CROSS_LOCK_INIT(), }
-
-/*
- * To initialize a lockdep_map statically use this macro.
- * Note that _name must not be NULL.
- */
-#define STATIC_LOCKDEP_MAP_INIT(_name, _key) \
- { .name = (_name), .key = (void *)(_key), .cross = 0, }
-
-extern void crossrelease_hist_start(enum xhlock_context_t c);
-extern void crossrelease_hist_end(enum xhlock_context_t c);
-extern void lockdep_invariant_state(bool force);
-extern void lockdep_init_task(struct task_struct *task);
-extern void lockdep_free_task(struct task_struct *task);
-#else /* !CROSSRELEASE */
#define lockdep_init_map_crosslock(m, n, k, s) do {} while (0)
/*
* To initialize a lockdep_map statically use this macro.
static inline void lockdep_invariant_state(bool force) {}
static inline void lockdep_init_task(struct task_struct *task) {}
static inline void lockdep_free_task(struct task_struct *task) {}
-#endif /* CROSSRELEASE */
#ifdef CONFIG_LOCK_STAT
*/
typedef struct {
arch_rwlock_t raw_lock;
-#ifdef CONFIG_GENERIC_LOCKBREAK
- unsigned int break_lock;
-#endif
#ifdef CONFIG_DEBUG_SPINLOCK
unsigned int magic, owner_cpu;
void *owner;
struct held_lock held_locks[MAX_LOCK_DEPTH];
#endif
-#ifdef CONFIG_LOCKDEP_CROSSRELEASE
-#define MAX_XHLOCKS_NR 64UL
- struct hist_lock *xhlocks; /* Crossrelease history locks */
- unsigned int xhlock_idx;
- /* For restoring at history boundaries */
- unsigned int xhlock_idx_hist[XHLOCK_CTX_NR];
- unsigned int hist_id;
- /* For overwrite check at each context exit */
- unsigned int hist_id_save[XHLOCK_CTX_NR];
-#endif
-
#ifdef CONFIG_UBSAN
unsigned int in_ubsan;
#endif
#define raw_spin_is_locked(lock) arch_spin_is_locked(&(lock)->raw_lock)
-#ifdef CONFIG_GENERIC_LOCKBREAK
-#define raw_spin_is_contended(lock) ((lock)->break_lock)
-#else
-
#ifdef arch_spin_is_contended
#define raw_spin_is_contended(lock) arch_spin_is_contended(&(lock)->raw_lock)
#else
#define raw_spin_is_contended(lock) (((void)(lock), 0))
#endif /*arch_spin_is_contended*/
-#endif
/*
* This barrier must provide two things:
typedef struct raw_spinlock {
arch_spinlock_t raw_lock;
-#ifdef CONFIG_GENERIC_LOCKBREAK
- unsigned int break_lock;
-#endif
#ifdef CONFIG_DEBUG_SPINLOCK
unsigned int magic, owner_cpu;
void *owner;
#define CREATE_TRACE_POINTS
#include <trace/events/lock.h>
-#ifdef CONFIG_LOCKDEP_CROSSRELEASE
-#include <linux/slab.h>
-#endif
-
#ifdef CONFIG_PROVE_LOCKING
int prove_locking = 1;
module_param(prove_locking, int, 0644);
#define lock_stat 0
#endif
-#ifdef CONFIG_BOOTPARAM_LOCKDEP_CROSSRELEASE_FULLSTACK
-static int crossrelease_fullstack = 1;
-#else
-static int crossrelease_fullstack;
-#endif
-static int __init allow_crossrelease_fullstack(char *str)
-{
- crossrelease_fullstack = 1;
- return 0;
-}
-
-early_param("crossrelease_fullstack", allow_crossrelease_fullstack);
-
/*
* lockdep_lock: protects the lockdep graph, the hashes and the
* class/list/hash allocators.
return is_static || static_obj(lock->key) ? NULL : ERR_PTR(-EINVAL);
}
-#ifdef CONFIG_LOCKDEP_CROSSRELEASE
-static void cross_init(struct lockdep_map *lock, int cross);
-static int cross_lock(struct lockdep_map *lock);
-static int lock_acquire_crosslock(struct held_lock *hlock);
-static int lock_release_crosslock(struct lockdep_map *lock);
-#else
-static inline void cross_init(struct lockdep_map *lock, int cross) {}
-static inline int cross_lock(struct lockdep_map *lock) { return 0; }
-static inline int lock_acquire_crosslock(struct held_lock *hlock) { return 2; }
-static inline int lock_release_crosslock(struct lockdep_map *lock) { return 2; }
-#endif
-
/*
* Register a lock's class in the hash-table, if the class is not present
* yet. Otherwise we look it up. We cache the result in the lock object
printk(KERN_CONT "\n\n");
}
- if (cross_lock(tgt->instance)) {
- printk(" Possible unsafe locking scenario by crosslock:\n\n");
- printk(" CPU0 CPU1\n");
- printk(" ---- ----\n");
- printk(" lock(");
- __print_lock_name(parent);
- printk(KERN_CONT ");\n");
- printk(" lock(");
- __print_lock_name(target);
- printk(KERN_CONT ");\n");
- printk(" lock(");
- __print_lock_name(source);
- printk(KERN_CONT ");\n");
- printk(" unlock(");
- __print_lock_name(target);
- printk(KERN_CONT ");\n");
- printk("\n *** DEADLOCK ***\n\n");
- } else {
- printk(" Possible unsafe locking scenario:\n\n");
- printk(" CPU0 CPU1\n");
- printk(" ---- ----\n");
- printk(" lock(");
- __print_lock_name(target);
- printk(KERN_CONT ");\n");
- printk(" lock(");
- __print_lock_name(parent);
- printk(KERN_CONT ");\n");
- printk(" lock(");
- __print_lock_name(target);
- printk(KERN_CONT ");\n");
- printk(" lock(");
- __print_lock_name(source);
- printk(KERN_CONT ");\n");
- printk("\n *** DEADLOCK ***\n\n");
- }
+ printk(" Possible unsafe locking scenario:\n\n");
+ printk(" CPU0 CPU1\n");
+ printk(" ---- ----\n");
+ printk(" lock(");
+ __print_lock_name(target);
+ printk(KERN_CONT ");\n");
+ printk(" lock(");
+ __print_lock_name(parent);
+ printk(KERN_CONT ");\n");
+ printk(" lock(");
+ __print_lock_name(target);
+ printk(KERN_CONT ");\n");
+ printk(" lock(");
+ __print_lock_name(source);
+ printk(KERN_CONT ");\n");
+ printk("\n *** DEADLOCK ***\n\n");
}
/*
curr->comm, task_pid_nr(curr));
print_lock(check_src);
- if (cross_lock(check_tgt->instance))
- pr_warn("\nbut now in release context of a crosslock acquired at the following:\n");
- else
- pr_warn("\nbut task is already holding lock:\n");
+ pr_warn("\nbut task is already holding lock:\n");
print_lock(check_tgt);
pr_warn("\nwhich lock already depends on the new lock.\n\n");
if (!debug_locks_off_graph_unlock() || debug_locks_silent)
return 0;
- if (cross_lock(check_tgt->instance))
- this->trace = *trace;
- else if (!save_trace(&this->trace))
+ if (!save_trace(&this->trace))
return 0;
depth = get_lock_depth(target);
if (nest)
return 2;
- if (cross_lock(prev->instance))
- continue;
-
return print_deadlock_bug(curr, prev, next);
}
return 1;
for (;;) {
int distance = curr->lockdep_depth - depth + 1;
hlock = curr->held_locks + depth - 1;
+
/*
- * Only non-crosslock entries get new dependencies added.
- * Crosslock entries will be added by commit later:
+ * Only non-recursive-read entries get new dependencies
+ * added:
*/
- if (!cross_lock(hlock->instance)) {
+ if (hlock->read != 2 && hlock->check) {
+ int ret = check_prev_add(curr, hlock, next, distance, &trace, save_trace);
+ if (!ret)
+ return 0;
+
/*
- * Only non-recursive-read entries get new dependencies
- * added:
+ * Stop after the first non-trylock entry,
+ * as non-trylock entries have added their
+ * own direct dependencies already, so this
+ * lock is connected to them indirectly:
*/
- if (hlock->read != 2 && hlock->check) {
- int ret = check_prev_add(curr, hlock, next,
- distance, &trace, save_trace);
- if (!ret)
- return 0;
-
- /*
- * Stop after the first non-trylock entry,
- * as non-trylock entries have added their
- * own direct dependencies already, so this
- * lock is connected to them indirectly:
- */
- if (!hlock->trylock)
- break;
- }
+ if (!hlock->trylock)
+ break;
}
+
depth--;
/*
* End of lock-stack?
void lockdep_init_map(struct lockdep_map *lock, const char *name,
struct lock_class_key *key, int subclass)
{
- cross_init(lock, 0);
__lockdep_init_map(lock, name, key, subclass);
}
EXPORT_SYMBOL_GPL(lockdep_init_map);
-#ifdef CONFIG_LOCKDEP_CROSSRELEASE
-void lockdep_init_map_crosslock(struct lockdep_map *lock, const char *name,
- struct lock_class_key *key, int subclass)
-{
- cross_init(lock, 1);
- __lockdep_init_map(lock, name, key, subclass);
-}
-EXPORT_SYMBOL_GPL(lockdep_init_map_crosslock);
-#endif
-
struct lock_class_key __lockdep_no_validate__;
EXPORT_SYMBOL_GPL(__lockdep_no_validate__);
int chain_head = 0;
int class_idx;
u64 chain_key;
- int ret;
if (unlikely(!debug_locks))
return 0;
class_idx = class - lock_classes + 1;
- /* TODO: nest_lock is not implemented for crosslock yet. */
- if (depth && !cross_lock(lock)) {
+ if (depth) {
hlock = curr->held_locks + depth - 1;
if (hlock->class_idx == class_idx && nest_lock) {
if (hlock->references) {
if (!validate_chain(curr, lock, hlock, chain_head, chain_key))
return 0;
- ret = lock_acquire_crosslock(hlock);
- /*
- * 2 means normal acquire operations are needed. Otherwise, it's
- * ok just to return with '0:fail, 1:success'.
- */
- if (ret != 2)
- return ret;
-
curr->curr_chain_key = chain_key;
curr->lockdep_depth++;
check_chain_key(curr);
struct task_struct *curr = current;
struct held_lock *hlock;
unsigned int depth;
- int ret, i;
+ int i;
if (unlikely(!debug_locks))
return 0;
- ret = lock_release_crosslock(lock);
- /*
- * 2 means normal release operations are needed. Otherwise, it's
- * ok just to return with '0:fail, 1:success'.
- */
- if (ret != 2)
- return ret;
-
depth = curr->lockdep_depth;
/*
* So we're all set to release this lock.. wait what lock? We don't
dump_stack();
}
EXPORT_SYMBOL_GPL(lockdep_rcu_suspicious);
-
-#ifdef CONFIG_LOCKDEP_CROSSRELEASE
-
-/*
- * Crossrelease works by recording a lock history for each thread and
- * connecting those historic locks that were taken after the
- * wait_for_completion() in the complete() context.
- *
- * Task-A Task-B
- *
- * mutex_lock(&A);
- * mutex_unlock(&A);
- *
- * wait_for_completion(&C);
- * lock_acquire_crosslock();
- * atomic_inc_return(&cross_gen_id);
- * |
- * | mutex_lock(&B);
- * | mutex_unlock(&B);
- * |
- * | complete(&C);
- * `-- lock_commit_crosslock();
- *
- * Which will then add a dependency between B and C.
- */
-
-#define xhlock(i) (current->xhlocks[(i) % MAX_XHLOCKS_NR])
-
-/*
- * Whenever a crosslock is held, cross_gen_id will be increased.
- */
-static atomic_t cross_gen_id; /* Can be wrapped */
-
-/*
- * Make an entry of the ring buffer invalid.
- */
-static inline void invalidate_xhlock(struct hist_lock *xhlock)
-{
- /*
- * Normally, xhlock->hlock.instance must be !NULL.
- */
- xhlock->hlock.instance = NULL;
-}
-
-/*
- * Lock history stacks; we have 2 nested lock history stacks:
- *
- * HARD(IRQ)
- * SOFT(IRQ)
- *
- * The thing is that once we complete a HARD/SOFT IRQ the future task locks
- * should not depend on any of the locks observed while running the IRQ. So
- * what we do is rewind the history buffer and erase all our knowledge of that
- * temporal event.
- */
-
-void crossrelease_hist_start(enum xhlock_context_t c)
-{
- struct task_struct *cur = current;
-
- if (!cur->xhlocks)
- return;
-
- cur->xhlock_idx_hist[c] = cur->xhlock_idx;
- cur->hist_id_save[c] = cur->hist_id;
-}
-
-void crossrelease_hist_end(enum xhlock_context_t c)
-{
- struct task_struct *cur = current;
-
- if (cur->xhlocks) {
- unsigned int idx = cur->xhlock_idx_hist[c];
- struct hist_lock *h = &xhlock(idx);
-
- cur->xhlock_idx = idx;
-
- /* Check if the ring was overwritten. */
- if (h->hist_id != cur->hist_id_save[c])
- invalidate_xhlock(h);
- }
-}
-
-/*
- * lockdep_invariant_state() is used to annotate independence inside a task, to
- * make one task look like multiple independent 'tasks'.
- *
- * Take for instance workqueues; each work is independent of the last. The
- * completion of a future work does not depend on the completion of a past work
- * (in general). Therefore we must not carry that (lock) dependency across
- * works.
- *
- * This is true for many things; pretty much all kthreads fall into this
- * pattern, where they have an invariant state and future completions do not
- * depend on past completions. Its just that since they all have the 'same'
- * form -- the kthread does the same over and over -- it doesn't typically
- * matter.
- *
- * The same is true for system-calls, once a system call is completed (we've
- * returned to userspace) the next system call does not depend on the lock
- * history of the previous system call.
- *
- * They key property for independence, this invariant state, is that it must be
- * a point where we hold no locks and have no history. Because if we were to
- * hold locks, the restore at _end() would not necessarily recover it's history
- * entry. Similarly, independence per-definition means it does not depend on
- * prior state.
- */
-void lockdep_invariant_state(bool force)
-{
- /*
- * We call this at an invariant point, no current state, no history.
- * Verify the former, enforce the latter.
- */
- WARN_ON_ONCE(!force && current->lockdep_depth);
- if (current->xhlocks)
- invalidate_xhlock(&xhlock(current->xhlock_idx));
-}
-
-static int cross_lock(struct lockdep_map *lock)
-{
- return lock ? lock->cross : 0;
-}
-
-/*
- * This is needed to decide the relationship between wrapable variables.
- */
-static inline int before(unsigned int a, unsigned int b)
-{
- return (int)(a - b) < 0;
-}
-
-static inline struct lock_class *xhlock_class(struct hist_lock *xhlock)
-{
- return hlock_class(&xhlock->hlock);
-}
-
-static inline struct lock_class *xlock_class(struct cross_lock *xlock)
-{
- return hlock_class(&xlock->hlock);
-}
-
-/*
- * Should we check a dependency with previous one?
- */
-static inline int depend_before(struct held_lock *hlock)
-{
- return hlock->read != 2 && hlock->check && !hlock->trylock;
-}
-
-/*
- * Should we check a dependency with next one?
- */
-static inline int depend_after(struct held_lock *hlock)
-{
- return hlock->read != 2 && hlock->check;
-}
-
-/*
- * Check if the xhlock is valid, which would be false if,
- *
- * 1. Has not used after initializaion yet.
- * 2. Got invalidated.
- *
- * Remind hist_lock is implemented as a ring buffer.
- */
-static inline int xhlock_valid(struct hist_lock *xhlock)
-{
- /*
- * xhlock->hlock.instance must be !NULL.
- */
- return !!xhlock->hlock.instance;
-}
-
-/*
- * Record a hist_lock entry.
- *
- * Irq disable is only required.
- */
-static void add_xhlock(struct held_lock *hlock)
-{
- unsigned int idx = ++current->xhlock_idx;
- struct hist_lock *xhlock = &xhlock(idx);
-
-#ifdef CONFIG_DEBUG_LOCKDEP
- /*
- * This can be done locklessly because they are all task-local
- * state, we must however ensure IRQs are disabled.
- */
- WARN_ON_ONCE(!irqs_disabled());
-#endif
-
- /* Initialize hist_lock's members */
- xhlock->hlock = *hlock;
- xhlock->hist_id = ++current->hist_id;
-
- xhlock->trace.nr_entries = 0;
- xhlock->trace.max_entries = MAX_XHLOCK_TRACE_ENTRIES;
- xhlock->trace.entries = xhlock->trace_entries;
-
- if (crossrelease_fullstack) {
- xhlock->trace.skip = 3;
- save_stack_trace(&xhlock->trace);
- } else {
- xhlock->trace.nr_entries = 1;
- xhlock->trace.entries[0] = hlock->acquire_ip;
- }
-}
-
-static inline int same_context_xhlock(struct hist_lock *xhlock)
-{
- return xhlock->hlock.irq_context == task_irq_context(current);
-}
-
-/*
- * This should be lockless as far as possible because this would be
- * called very frequently.
- */
-static void check_add_xhlock(struct held_lock *hlock)
-{
- /*
- * Record a hist_lock, only in case that acquisitions ahead
- * could depend on the held_lock. For example, if the held_lock
- * is trylock then acquisitions ahead never depends on that.
- * In that case, we don't need to record it. Just return.
- */
- if (!current->xhlocks || !depend_before(hlock))
- return;
-
- add_xhlock(hlock);
-}
-
-/*
- * For crosslock.
- */
-static int add_xlock(struct held_lock *hlock)
-{
- struct cross_lock *xlock;
- unsigned int gen_id;
-
- if (!graph_lock())
- return 0;
-
- xlock = &((struct lockdep_map_cross *)hlock->instance)->xlock;
-
- /*
- * When acquisitions for a crosslock are overlapped, we use
- * nr_acquire to perform commit for them, based on cross_gen_id
- * of the first acquisition, which allows to add additional
- * dependencies.
- *
- * Moreover, when no acquisition of a crosslock is in progress,
- * we should not perform commit because the lock might not exist
- * any more, which might cause incorrect memory access. So we
- * have to track the number of acquisitions of a crosslock.
- *
- * depend_after() is necessary to initialize only the first
- * valid xlock so that the xlock can be used on its commit.
- */
- if (xlock->nr_acquire++ && depend_after(&xlock->hlock))
- goto unlock;
-
- gen_id = (unsigned int)atomic_inc_return(&cross_gen_id);
- xlock->hlock = *hlock;
- xlock->hlock.gen_id = gen_id;
-unlock:
- graph_unlock();
- return 1;
-}
-
-/*
- * Called for both normal and crosslock acquires. Normal locks will be
- * pushed on the hist_lock queue. Cross locks will record state and
- * stop regular lock_acquire() to avoid being placed on the held_lock
- * stack.
- *
- * Return: 0 - failure;
- * 1 - crosslock, done;
- * 2 - normal lock, continue to held_lock[] ops.
- */
-static int lock_acquire_crosslock(struct held_lock *hlock)
-{
- /*
- * CONTEXT 1 CONTEXT 2
- * --------- ---------
- * lock A (cross)
- * X = atomic_inc_return(&cross_gen_id)
- * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
- * Y = atomic_read_acquire(&cross_gen_id)
- * lock B
- *
- * atomic_read_acquire() is for ordering between A and B,
- * IOW, A happens before B, when CONTEXT 2 see Y >= X.
- *
- * Pairs with atomic_inc_return() in add_xlock().
- */
- hlock->gen_id = (unsigned int)atomic_read_acquire(&cross_gen_id);
-
- if (cross_lock(hlock->instance))
- return add_xlock(hlock);
-
- check_add_xhlock(hlock);
- return 2;
-}
-
-static int copy_trace(struct stack_trace *trace)
-{
- unsigned long *buf = stack_trace + nr_stack_trace_entries;
- unsigned int max_nr = MAX_STACK_TRACE_ENTRIES - nr_stack_trace_entries;
- unsigned int nr = min(max_nr, trace->nr_entries);
-
- trace->nr_entries = nr;
- memcpy(buf, trace->entries, nr * sizeof(trace->entries[0]));
- trace->entries = buf;
- nr_stack_trace_entries += nr;
-
- if (nr_stack_trace_entries >= MAX_STACK_TRACE_ENTRIES-1) {
- if (!debug_locks_off_graph_unlock())
- return 0;
-
- print_lockdep_off("BUG: MAX_STACK_TRACE_ENTRIES too low!");
- dump_stack();
-
- return 0;
- }
-
- return 1;
-}
-
-static int commit_xhlock(struct cross_lock *xlock, struct hist_lock *xhlock)
-{
- unsigned int xid, pid;
- u64 chain_key;
-
- xid = xlock_class(xlock) - lock_classes;
- chain_key = iterate_chain_key((u64)0, xid);
- pid = xhlock_class(xhlock) - lock_classes;
- chain_key = iterate_chain_key(chain_key, pid);
-
- if (lookup_chain_cache(chain_key))
- return 1;
-
- if (!add_chain_cache_classes(xid, pid, xhlock->hlock.irq_context,
- chain_key))
- return 0;
-
- if (!check_prev_add(current, &xlock->hlock, &xhlock->hlock, 1,
- &xhlock->trace, copy_trace))
- return 0;
-
- return 1;
-}
-
-static void commit_xhlocks(struct cross_lock *xlock)
-{
- unsigned int cur = current->xhlock_idx;
- unsigned int prev_hist_id = xhlock(cur).hist_id;
- unsigned int i;
-
- if (!graph_lock())
- return;
-
- if (xlock->nr_acquire) {
- for (i = 0; i < MAX_XHLOCKS_NR; i++) {
- struct hist_lock *xhlock = &xhlock(cur - i);
-
- if (!xhlock_valid(xhlock))
- break;
-
- if (before(xhlock->hlock.gen_id, xlock->hlock.gen_id))
- break;
-
- if (!same_context_xhlock(xhlock))
- break;
-
- /*
- * Filter out the cases where the ring buffer was
- * overwritten and the current entry has a bigger
- * hist_id than the previous one, which is impossible
- * otherwise:
- */
- if (unlikely(before(prev_hist_id, xhlock->hist_id)))
- break;
-
- prev_hist_id = xhlock->hist_id;
-
- /*
- * commit_xhlock() returns 0 with graph_lock already
- * released if fail.
- */
- if (!commit_xhlock(xlock, xhlock))
- return;
- }
- }
-
- graph_unlock();
-}
-
-void lock_commit_crosslock(struct lockdep_map *lock)
-{
- struct cross_lock *xlock;
- unsigned long flags;
-
- if (unlikely(!debug_locks || current->lockdep_recursion))
- return;
-
- if (!current->xhlocks)
- return;
-
- /*
- * Do commit hist_locks with the cross_lock, only in case that
- * the cross_lock could depend on acquisitions after that.
- *
- * For example, if the cross_lock does not have the 'check' flag
- * then we don't need to check dependencies and commit for that.
- * Just skip it. In that case, of course, the cross_lock does
- * not depend on acquisitions ahead, either.
- *
- * WARNING: Don't do that in add_xlock() in advance. When an
- * acquisition context is different from the commit context,
- * invalid(skipped) cross_lock might be accessed.
- */
- if (!depend_after(&((struct lockdep_map_cross *)lock)->xlock.hlock))
- return;
-
- raw_local_irq_save(flags);
- check_flags(flags);
- current->lockdep_recursion = 1;
- xlock = &((struct lockdep_map_cross *)lock)->xlock;
- commit_xhlocks(xlock);
- current->lockdep_recursion = 0;
- raw_local_irq_restore(flags);
-}
-EXPORT_SYMBOL_GPL(lock_commit_crosslock);
-
-/*
- * Return: 0 - failure;
- * 1 - crosslock, done;
- * 2 - normal lock, continue to held_lock[] ops.
- */
-static int lock_release_crosslock(struct lockdep_map *lock)
-{
- if (cross_lock(lock)) {
- if (!graph_lock())
- return 0;
- ((struct lockdep_map_cross *)lock)->xlock.nr_acquire--;
- graph_unlock();
- return 1;
- }
- return 2;
-}
-
-static void cross_init(struct lockdep_map *lock, int cross)
-{
- if (cross)
- ((struct lockdep_map_cross *)lock)->xlock.nr_acquire = 0;
-
- lock->cross = cross;
-
- /*
- * Crossrelease assumes that the ring buffer size of xhlocks
- * is aligned with power of 2. So force it on build.
- */
- BUILD_BUG_ON(MAX_XHLOCKS_NR & (MAX_XHLOCKS_NR - 1));
-}
-
-void lockdep_init_task(struct task_struct *task)
-{
- int i;
-
- task->xhlock_idx = UINT_MAX;
- task->hist_id = 0;
-
- for (i = 0; i < XHLOCK_CTX_NR; i++) {
- task->xhlock_idx_hist[i] = UINT_MAX;
- task->hist_id_save[i] = 0;
- }
-
- task->xhlocks = kzalloc(sizeof(struct hist_lock) * MAX_XHLOCKS_NR,
- GFP_KERNEL);
-}
-
-void lockdep_free_task(struct task_struct *task)
-{
- if (task->xhlocks) {
- void *tmp = task->xhlocks;
- /* Diable crossrelease for current */
- task->xhlocks = NULL;
- kfree(tmp);
- }
-}
-#endif
break; \
preempt_enable(); \
\
- if (!(lock)->break_lock) \
- (lock)->break_lock = 1; \
- while ((lock)->break_lock) \
- arch_##op##_relax(&lock->raw_lock); \
+ arch_##op##_relax(&lock->raw_lock); \
} \
- (lock)->break_lock = 0; \
} \
\
unsigned long __lockfunc __raw_##op##_lock_irqsave(locktype##_t *lock) \
local_irq_restore(flags); \
preempt_enable(); \
\
- if (!(lock)->break_lock) \
- (lock)->break_lock = 1; \
- while ((lock)->break_lock) \
- arch_##op##_relax(&lock->raw_lock); \
+ arch_##op##_relax(&lock->raw_lock); \
} \
- (lock)->break_lock = 0; \
+ \
return flags; \
} \
\
select DEBUG_MUTEXES
select DEBUG_RT_MUTEXES if RT_MUTEXES
select DEBUG_LOCK_ALLOC
- select LOCKDEP_CROSSRELEASE
- select LOCKDEP_COMPLETIONS
select TRACE_IRQFLAGS
default n
help
CONFIG_LOCK_STAT defines "contended" and "acquired" lock events.
(CONFIG_LOCKDEP defines "acquire" and "release" events.)
-config LOCKDEP_CROSSRELEASE
- bool
- help
- This makes lockdep work for crosslock which is a lock allowed to
- be released in a different context from the acquisition context.
- Normally a lock must be released in the context acquiring the lock.
- However, relexing this constraint helps synchronization primitives
- such as page locks or completions can use the lock correctness
- detector, lockdep.
-
-config LOCKDEP_COMPLETIONS
- bool
- help
- A deadlock caused by wait_for_completion() and complete() can be
- detected by lockdep using crossrelease feature.
-
-config BOOTPARAM_LOCKDEP_CROSSRELEASE_FULLSTACK
- bool "Enable the boot parameter, crossrelease_fullstack"
- depends on LOCKDEP_CROSSRELEASE
- default n
- help
- The lockdep "cross-release" feature needs to record stack traces
- (of calling functions) for all acquisitions, for eventual later
- use during analysis. By default only a single caller is recorded,
- because the unwind operation can be very expensive with deeper
- stack chains.
-
- However a boot parameter, crossrelease_fullstack, was
- introduced since sometimes deeper traces are required for full
- analysis. This option turns on the boot parameter.
-
config DEBUG_LOCKDEP
bool "Lock dependency engine debugging"
depends on DEBUG_KERNEL && LOCKDEP
}
}
-# whine about ACCESS_ONCE
- if ($^V && $^V ge 5.10.0 &&
- $line =~ /\bACCESS_ONCE\s*$balanced_parens\s*(=(?!=))?\s*($FuncArg)?/) {
- my $par = $1;
- my $eq = $2;
- my $fun = $3;
- $par =~ s/^\(\s*(.*)\s*\)$/$1/;
- if (defined($eq)) {
- if (WARN("PREFER_WRITE_ONCE",
- "Prefer WRITE_ONCE(<FOO>, <BAR>) over ACCESS_ONCE(<FOO>) = <BAR>\n" . $herecurr) &&
- $fix) {
- $fixed[$fixlinenr] =~ s/\bACCESS_ONCE\s*\(\s*\Q$par\E\s*\)\s*$eq\s*\Q$fun\E/WRITE_ONCE($par, $fun)/;
- }
- } else {
- if (WARN("PREFER_READ_ONCE",
- "Prefer READ_ONCE(<FOO>) over ACCESS_ONCE(<FOO>)\n" . $herecurr) &&
- $fix) {
- $fixed[$fixlinenr] =~ s/\bACCESS_ONCE\s*\(\s*\Q$par\E\s*\)/READ_ONCE($par)/;
- }
- }
- }
-
# check for mutex_trylock_recursive usage
if ($line =~ /mutex_trylock_recursive/) {
ERROR("LOCKING",
#define uninitialized_var(x) x = *(&(x))
-#define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x))
-
#include <linux/types.h>
/*
/*
* Prevent the compiler from merging or refetching reads or writes. The
* compiler is also forbidden from reordering successive instances of
- * READ_ONCE, WRITE_ONCE and ACCESS_ONCE (see below), but only when the
- * compiler is aware of some particular ordering. One way to make the
- * compiler aware of ordering is to put the two invocations of READ_ONCE,
- * WRITE_ONCE or ACCESS_ONCE() in different C statements.
+ * READ_ONCE and WRITE_ONCE, but only when the compiler is aware of some
+ * particular ordering. One way to make the compiler aware of ordering is to
+ * put the two invocations of READ_ONCE or WRITE_ONCE in different C
+ * statements.
*
- * In contrast to ACCESS_ONCE these two macros will also work on aggregate
- * data types like structs or unions. If the size of the accessed data
- * type exceeds the word size of the machine (e.g., 32 bits or 64 bits)
- * READ_ONCE() and WRITE_ONCE() will fall back to memcpy and print a
- * compile-time warning.
+ * These two macros will also work on aggregate data types like structs or
+ * unions. If the size of the accessed data type exceeds the word size of
+ * the machine (e.g., 32 bits or 64 bits) READ_ONCE() and WRITE_ONCE() will
+ * fall back to memcpy and print a compile-time warning.
*
* Their two major use cases are: (1) Mediating communication between
* process-level code and irq/NMI handlers, all running on the same CPU,
- * and (2) Ensuring that the compiler does not fold, spindle, or otherwise
+ * and (2) Ensuring that the compiler does not fold, spindle, or otherwise
* mutilate accesses that either do not require ordering or that interact
* with an explicit memory barrier or atomic instruction that provides the
* required ordering.
#define printk(...) dprintf(STDOUT_FILENO, __VA_ARGS__)
#define pr_err(format, ...) fprintf (stderr, format, ## __VA_ARGS__)
#define pr_warn pr_err
+#define pr_cont pr_err
#define list_del_rcu list_del
static inline u64 perf_mmap__read_head(struct perf_mmap *mm)
{
struct perf_event_mmap_page *pc = mm->base;
- u64 head = ACCESS_ONCE(pc->data_head);
+ u64 head = READ_ONCE(pc->data_head);
rmb();
return head;
}
git.samba.org / sfrench / cifs-2.6.git / commitdiff