Re: [PATCH] locking: Document that mutex_unlock() is non-atomic

[Waiman Long <longman@redhat.com>]

On 11/30/23 15:48, Jann Horn wrote:
> 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.
> My understanding is that 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, I think; 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.)
>
> If my understanding of this is correct, we should probably document this -
> I think such a semantic difference between mutexes and spinlocks is fairly
> unintuitive.

Spinlocks are fair. So doing a lock/unlock sequence will make sure that 
all the previously waiting waiters are done with the lock. Para-virtual 
spinlocks, however, can be a bit unfair so doing a lock/unlock sequence 
may not be enough to guarantee there is no waiter. The same is true for 
mutex. Adding a spin_is_locked() or mutex_is_locked() check can make 
sure that all the waiters are gone.

Also the term "non-atomc" is kind of ambiguous as to what is the exact 
meaning of this word.

Cheers,
Longman

>
> Signed-off-by: Jann Horn <jannh@google.com>
> ---
> I hope for some thorough review on this patch to make sure the comments
> I'm adding are actually true, and to confirm that mutexes intentionally
> do not support this usage pattern.
>
>   Documentation/locking/mutex-design.rst | 6 ++++++
>   kernel/locking/mutex.c                 | 5 +++++
>   2 files changed, 11 insertions(+)
>
> diff --git a/Documentation/locking/mutex-design.rst b/Documentation/locking/mutex-design.rst
> index 78540cd7f54b..087716bfa7b2 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 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
>   ----------
> diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
> index 2deeeca3e71b..4c6b83bab643 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 in this aspect.
> + *
>    * This function is similar to (but not equivalent to) up().
>    */
>   void __sched mutex_unlock(struct mutex *lock)
>
> base-commit: 3b47bc037bd44f142ac09848e8d3ecccc726be99