diff options
| 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 /kernel/locking | |
| parent | 5431fdd2c181dd2eac218e45b44deb2925fa48f0 (diff) | |
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 'kernel/locking')
| -rw-r--r-- | kernel/locking/mutex.c | 5 |
1 files changed, 5 insertions, 0 deletions
diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c index 2deeeca3e71b..cbae8c0b89ab 100644 --- a/kernel/locking/mutex.c +++ b/kernel/locking/mutex.c @@ -532,6 +532,11 @@ static noinline void __sched __mutex_unlock_slowpath(struct mutex *lock, unsigne * This function must not be used in interrupt context. Unlocking * of a not locked mutex is not allowed. * + * The caller must ensure that the mutex stays alive until this function has + * returned - mutex_unlock() can NOT directly be used to release an object such + * that another concurrent task can free it. + * Mutexes are different from spinlocks & refcounts in this aspect. + * * This function is similar to (but not equivalent to) up(). */ void __sched mutex_unlock(struct mutex *lock) |