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

[David Laight <David.Laight@ACULAB.COM>]

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

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