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| author | Jann Horn <jannh@google.com> | 2023-11-30 21:48:17 +0100 |
|---|---|---|
| committer | Ingo Molnar <mingo@kernel.org> | 2023-12-01 11:27:43 +0100 |
| commit | a51749ab34d9e5dec548fe38ede7e01e8bb26454 (patch) | |
| tree | 74b1c1455396b45b89516a2ce89d2cc843dfe735 /Documentation/locking | |
| parent | 5431fdd2c181dd2eac218e45b44deb2925fa48f0 (diff) | |
| download | lwn-a51749ab34d9e5dec548fe38ede7e01e8bb26454.tar.gz lwn-a51749ab34d9e5dec548fe38ede7e01e8bb26454.zip | |
locking/mutex: Document that mutex_unlock() is non-atomic
I have seen several cases of attempts to use mutex_unlock() to release an
object such that the object can then be freed by another task.
This is not safe because mutex_unlock(), in the
MUTEX_FLAG_WAITERS && !MUTEX_FLAG_HANDOFF case, accesses the mutex
structure after having marked it as unlocked; so mutex_unlock() requires
its caller to ensure that the mutex stays alive until mutex_unlock()
returns.
If MUTEX_FLAG_WAITERS is set and there are real waiters, those waiters
have to keep the mutex alive, but we could have a spurious
MUTEX_FLAG_WAITERS left if an interruptible/killable waiter bailed
between the points where __mutex_unlock_slowpath() did the cmpxchg
reading the flags and where it acquired the wait_lock.
( With spinlocks, that kind of code pattern is allowed and, from what I
remember, used in several places in the kernel. )
Document this, such a semantic difference between mutexes and spinlocks
is fairly unintuitive.
[ mingo: Made the changelog a bit more assertive, refined the comments. ]
Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20231130204817.2031407-1-jannh@google.com
Diffstat (limited to 'Documentation/locking')
| -rw-r--r-- | Documentation/locking/mutex-design.rst | 6 |
1 files changed, 6 insertions, 0 deletions
diff --git a/Documentation/locking/mutex-design.rst b/Documentation/locking/mutex-design.rst index 78540cd7f54b..7572339b2f12 100644 --- a/Documentation/locking/mutex-design.rst +++ b/Documentation/locking/mutex-design.rst @@ -101,6 +101,12 @@ 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. Interfaces ---------- |