linux-kernel - Re: [PATCH] locking: Document that mutex

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Date:   Thu, 30 Nov 2023 23:24:23 +0100
From:   Jann Horn <jannh@...gle.com>
To:     Waiman Long <longman@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Jonathan Corbet <corbet@....net>, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org
Subject: Re: [PATCH] locking: Document that mutex_unlock() is non-atomic

On Thu, Nov 30, 2023 at 10:53 PM Waiman Long <longman@...hat.com> 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.
>
> Could you clarify under what condition a concurrent task can decide to
> free the object holding the mutex? Is it !mutex_is_locked() or after a
> mutex_lock()/mutex_unlock sequence?

I mean a mutex_lock()+mutex_unlock() sequence.

> mutex_is_locked() will return true if the mutex has waiter even if it
> is currently free.

I don't understand your point, and maybe I also don't understand what
you mean by "free". Isn't mutex_is_locked() defined such that it only
looks at whether a mutex has an owner, and doesn't look at the waiter
list?