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@xxxxxxxxxxxxxxxxx>
---
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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