locking/qspinlock: Rework some comments

While working my way through the code again; I felt the comments could
use help.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Will Deacon <will.deacon@arm.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: andrea.parri@amarulasolutions.com
Cc: longman@redhat.com
Link: https://lkml.kernel.org/r/20181003130257.156322446@infradead.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
index ec343276f975e92388cd0759f248903d04363da3..47cb99787e4d94d053c847cf0d0a0b1a50cf0756 100644 (file)
--- a/kernel/locking/qspinlock.c
+++ b/kernel/locking/qspinlock.c
@@ -326,16 +326,23 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
        /*
         * trylock || pending
         *
-        * 0,0,0 -> 0,0,1 ; trylock
-        * 0,0,1 -> 0,1,1 ; pending
+        * 0,0,* -> 0,1,* -> 0,0,1 pending, trylock
         */
        val = atomic_fetch_or_acquire(_Q_PENDING_VAL, &lock->val);
+
        /*
-        * If we observe any contention; undo and queue.
+        * If we observe contention, there is a concurrent locker.
+        *
+        * Undo and queue; our setting of PENDING might have made the
+        * n,0,0 -> 0,0,0 transition fail and it will now be waiting
+        * on @next to become !NULL.
         */
        if (unlikely(val & ~_Q_LOCKED_MASK)) {
+
+               /* Undo PENDING if we set it. */
                if (!(val & _Q_PENDING_MASK))
                        clear_pending(lock);
+
                goto queue;
        }
 
@@ -474,16 +481,25 @@ locked:
         */
 
        /*
-        * In the PV case we might already have _Q_LOCKED_VAL set.
+        * In the PV case we might already have _Q_LOCKED_VAL set, because
+        * of lock stealing; therefore we must also allow:
         *
-        * The atomic_cond_read_acquire() call above has provided the
-        * necessary acquire semantics required for locking.
+        * n,0,1 -> 0,0,1
+        *
+        * Note: at this point: (val & _Q_PENDING_MASK) == 0, because of the
+        *       above wait condition, therefore any concurrent setting of
+        *       PENDING will make the uncontended transition fail.
         */
-       if (((val & _Q_TAIL_MASK) == tail) &&
-           atomic_try_cmpxchg_relaxed(&lock->val, &val, _Q_LOCKED_VAL))
-               goto release; /* No contention */
+       if ((val & _Q_TAIL_MASK) == tail) {
+               if (atomic_try_cmpxchg_relaxed(&lock->val, &val, _Q_LOCKED_VAL))
+                       goto release; /* No contention */
+       }
 
-       /* Either somebody is queued behind us or _Q_PENDING_VAL is set */
+       /*
+        * Either somebody is queued behind us or _Q_PENDING_VAL got set
+        * which will then detect the remaining tail and queue behind us
+        * ensuring we'll see a @next.
+        */
        set_locked(lock);
 
        /*