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

[David Laight]

From: Jann Horn
> Sent: 01 December 2023 15:02
> 
> On Fri, Dec 1, 2023 at 1:33 AM Waiman Long <longman@xxxxxxxxxx> wrote:
> > 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.
> 
> I think this pattern anyway only works when you're only trying to wait
> for the current holder of the lock, not tasks that are queued up on
> the lock as waiters - so a task initially holds a stable reference to
> some object, then acquires the object's lock, then drops the original
> reference, and then later drops the lock.
> You can see an example of such mutex usage (which is explicitly legal
> with userspace POSIX mutexes, but is forbidden with kernel mutexes) at
> the bottom of the POSIX manpage for pthread_mutex_destroy() at
> <https://pubs.opengroup.org/onlinepubs/007904875/functions/pthread_mutex_destroy.html>,
> in the section "Destroying Mutexes".

I don't understand at all what any of this is about.
You cannot de-initialise, free (etc) a mutex (or any other piece of
memory for that matter) if another thread can have a reference to it.
If some other code might be holding the mutex it also might be just
about to acquire it - you always need another lock of some kind to
ensure that doesn't happen.

IIRC pretty much the only time you need to acquire the mutex in the
free path is if locks are chained, eg:
	lock(table)
	entry = find_entry();
	lock(entry)
	unlock(table)
	...
	unlock(entry)

Then the free code has to:
	lock(table)
	remove_from_table(entry)
	lock(entry)
	unlock(entry)
	unlock(table)
	free(entry)

ISTR something about mutex_unlock() not being a full memory barrier.
But that is entirely different to this discussion.

	David

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