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Message-ID: <tkrat.12a11fb05527c6ed@s5r6.in-berlin.de>
Date: Thu, 19 Aug 2010 00:08:15 +0200 (CEST)
From: Stefan Richter <stefanr@...6.in-berlin.de>
To: linux-kernel@...r.kernel.org,
Rusty Russell <rusty@...tcorp.com.au>,
Matthew Wilcox <matthew@....cx>
cc: Randy Dunlap <rdunlap@...otime.net>, linux-doc@...r.kernel.org
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@...6.in-berlin.de>
---
Documentation/DocBook/kernel-locking.tmpl | 7 +++++--
1 file changed, 5 insertions(+), 2 deletions(-)
Index: b/Documentation/DocBook/kernel-locking.tmpl
===================================================================
--- 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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