[tip: locking/core] locking/mutex: Clarify that mutex_unlock(), and most other sleeping locks, can still use the lock object after it's unlocked

tip-bot2 for Ingo Molnar
2024-01-08T09:01:11Z

The following commit has been merged into the locking/core branch of tip:

Commit-ID: 2b9d9e0a9ba0e24cb9c78336481f0ed8b2bc1ff2
Gitweb: https://git.kernel.org/tip/2b9d9e0a9ba0e24cb9c78336481f0ed8b2bc1ff2
Author: Ingo Molnar <mingo@kernel.org>
AuthorDate: Mon, 08 Jan 2024 09:31:16 +01:00
Committer: Ingo Molnar <mingo@kernel.org>
CommitterDate: Mon, 08 Jan 2024 09:55:31 +01:00

locking/mutex: Clarify that mutex_unlock(), and most other sleeping locks, can still use the lock object after it's unlocked

Clarify the mutex lock lifetime rules a bit more.

Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Jann Horn <jannh@google.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/20231201121808.GL3818@noisy.programming.kicks-ass.net
---
Documentation/locking/mutex-design.rst | 24 ++++++++++++++++++------
1 file changed, 18 insertions(+), 6 deletions(-)

diff --git a/Documentation/locking/mutex-design.rst b/Documentation/locking/mutex-design.rst
index 7572339..7c30b4a 100644
--- a/Documentation/locking/mutex-design.rst
+++ b/Documentation/locking/mutex-design.rst
@@ -101,12 +101,24 @@ features that make lock debugging easier and faster:
- Detects multi-task circular deadlocks and prints out all affected
locks and tasks (and only those tasks).

-Releasing a mutex is not an atomic operation: Once a mutex release operation
-has begun, another context may be able to acquire the mutex before the release
-operation has fully completed. The mutex user must ensure that the mutex is not
-destroyed while a release operation is still in progress - in other words,
-callers of mutex_unlock() must ensure that the mutex stays alive until
-mutex_unlock() has returned.
+Mutexes - and most other sleeping locks like rwsems - do not provide an
+implicit reference for the memory they occupy, which reference is released
+with mutex_unlock().
+
+[ This is in contrast with spin_unlock() [or completion_done()], which
+ APIs can be used to guarantee that the memory is not touched by the
+ lock implementation after spin_unlock()/completion_done() releases
+ the lock. ]
+
+mutex_unlock() may access the mutex structure even after it has internally
+released the lock already - so it's not safe for another context to
+acquire the mutex and assume that the mutex_unlock() context is not using
+the structure anymore.
+
+The mutex user must ensure that the mutex is not destroyed while a
+release operation is still in progress - in other words, callers of
+mutex_unlock() must ensure that the mutex stays alive until mutex_unlock()
+has returned.

Interfaces
----------