Documentation: kernel-locking: mutex_trylock cannot be used in interrupt context

Chapter 6 is right about mutex_trylock, but chapter 10 wasn't.  This error
was introduced during semaphore-to-mutex conversion of the Unreliable
guide.  :-)

If user context which performs mutex_lock() or mutex_trylock() is
preempted by interrupt context which performs mutex_trylock() on the same
mutex instance, a deadlock occurs.  This is because these functions do not
disable local IRQs when they operate on mutex->wait_lock.

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
Acked-by: Rusty Russell <rusty@rustcorp.com.au>
Cc: Matthew Wilcox <matthew@wil.cx>
Cc: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/DocBook/kernel-locking.tmpl

@@ -1922,9 +1922,12 @@ machines due to caching.
       <function>mutex_lock()</function>
       </para>
       <para>
-       There is a <function>mutex_trylock()</function> which can be
-       used inside interrupt context, as it will not sleep.
+       There is a <function>mutex_trylock()</function> which does not
+       sleep.  Still, it must not be used inside interrupt context since
+       its implementation is not safe for that.
        <function>mutex_unlock()</function> will also never sleep.
+       It cannot be used in interrupt context either since a mutex
+       must be released by the same task that acquired it.
       </para>
      </listitem>
     </itemizedlist>