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

Waiman Long
2023-12-02T22:39:47Z

On 12/2/23 10:51, David Laight wrote:
> From: Waiman Long
>> Sent: 01 December 2023 19:16
>>
>> On 12/1/23 13:44, David Laight wrote:
>>> Pending waiters aren't the problem.
>>>
>> Pending waiters can still be a problem if code decides to free the lock
>> containing object after a lock/unlock sequence as it may cause
>> use-after-free.
>>> You have to ensure there aren't any, but the mutex() can be held.
>>>
>> Using reference count to track the number of active users is one way to
>> prevent that if you only release the reference count after
>> mutex_unlock() returns but not in the lock critical section.
> I suspect the documentation need to be more explicit than just saying
> it is non-atomic.
> Saying something like:
>
> The mutex structure may be accessed by mutex_unlock() after another
> thread has locked and unlocked the mutex.
>
> So if a reference count is used to ensure a structure remains valid when
> a lock is released (with the item being freed when the count becomes zero)
> the reference count itself cannot be protected by a mutex in the structure.
> So code like:
> ...
> count = --item->refcount;
> mutex_unlock(item->mtx);
> if (!count)
> free(item);
> can lead to a 'use after free' in mutex_unlock().
> However if the refcount is atomic and decremented without the
> mutex held there isn't a problem.
>
> David

That is definitely better than saying it is non-atomic which is vague in
meaning.

Cheers,
Longman