Merge branch 'master' of git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux-2.6

[sfrench/cifs-2.6.git] / Documentation / DocBook / kernel-locking.tmpl

diff --git a/Documentation/DocBook/kernel-locking.tmpl b/Documentation/DocBook/kernel-locking.tmpl
index 2e9d6b41f034594b3b1af87c97ab1da81f5caccd..2510763295d09dfbd893e288d1948c9b05feda4d 100644 (file)
--- a/Documentation/DocBook/kernel-locking.tmpl
+++ b/Documentation/DocBook/kernel-locking.tmpl
@@ -241,7 +241,7 @@
    </para>
    <para>
      The third type is a semaphore
-     (<filename class="headerfile">include/asm/semaphore.h</filename>): it
+     (<filename class="headerfile">include/linux/semaphore.h</filename>): it
      can have more than one holder at any time (the number decided at
      initialization time), although it is most commonly used as a
      single-holder lock (a mutex).  If you can't get a semaphore, your
@@ -290,7 +290,7 @@
      <para>
        If you have a data structure which is only ever accessed from
        user context, then you can use a simple semaphore
-       (<filename>linux/asm/semaphore.h</filename>) to protect it.  This 
+       (<filename>linux/linux/semaphore.h</filename>) to protect it.  This
        is the most trivial case: you initialize the semaphore to the number 
        of resources available (usually 1), and call
        <function>down_interruptible()</function> to grab the semaphore, and 
@@ -703,6 +703,31 @@
 </sect1>
 </chapter>
 
+<chapter id="trylock-functions">
+ <title>The trylock Functions</title>
+  <para>
+   There are functions that try to acquire a lock only once and immediately
+   return a value telling about success or failure to acquire the lock.
+   They can be used if you need no access to the data protected with the lock
+   when some other thread is holding the lock. You should acquire the lock
+   later if you then need access to the data protected with the lock.
+  </para>
+
+  <para>
+    <function>spin_trylock()</function> does not spin but returns non-zero if
+    it acquires the spinlock on the first try or 0 if not. This function can
+    be used in all contexts like <function>spin_lock</function>: you must have
+    disabled the contexts that might interrupt you and acquire the spin lock.
+  </para>
+
+  <para>
+    <function>mutex_trylock()</function> does not suspend your task
+    but returns non-zero if it could lock the mutex on the first try
+    or 0 if not. This function cannot be safely used in hardware or software
+    interrupt contexts despite not sleeping.
+  </para>
+</chapter>
+
   <chapter id="Examples">
    <title>Common Examples</title>
     <para>
@@ -854,7 +879,7 @@ The change is shown below, in standard patch format: the
  };
 
 -static DEFINE_MUTEX(cache_lock);
-+static spinlock_t cache_lock = SPIN_LOCK_UNLOCKED;
++static DEFINE_SPINLOCK(cache_lock);
  static LIST_HEAD(cache);
  static unsigned int cache_num = 0;
  #define MAX_CACHE_SIZE 10
@@ -1238,7 +1263,7 @@ Here is the "lock-per-object" implementation:
 -        int popularity;
  };
 
- static spinlock_t cache_lock = SPIN_LOCK_UNLOCKED;
+ static DEFINE_SPINLOCK(cache_lock);
 @@ -77,6 +84,7 @@
          obj-&gt;id = id;
          obj-&gt;popularity = 0;
@@ -1656,7 +1681,7 @@ the amount of locking which needs to be done.
  #include &lt;linux/slab.h&gt;
  #include &lt;linux/string.h&gt;
 +#include &lt;linux/rcupdate.h&gt;
- #include &lt;asm/semaphore.h&gt;
+ #include &lt;linux/semaphore.h&gt;
  #include &lt;asm/errno.h&gt;
 
  struct object