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

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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>
This commit is contained in:
Stefan Richter 2010-08-19 14:13:43 -07:00 committed by Linus Torvalds
parent 626115cda9
commit 1ee4168057
1 changed files with 5 additions and 2 deletions
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7
Documentation/DocBook/kernel-locking.tmpl
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@ -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>