From 1ee41680572971e34d90d5f584daf33195ec6dcb Mon Sep 17 00:00:00 2001
From: Stefan Richter <stefanr@s5r6.in-berlin.de>
Date: Thu, 19 Aug 2010 14:13:43 -0700
Subject: [PATCH] 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 | 7 +++++--
1 file changed, 5 insertions(+), 2 deletions(-)
diff --git a/Documentation/DocBook/kernel-locking.tmpl b/Documentation/DocBook/kernel-locking.tmpl
index 084f6ad7b7a0..0b1a3f97f285 100644
--- a/Documentation/DocBook/kernel-locking.tmpl
+++ b/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>
--
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