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

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Message-ID: <c7c62df7-74bb-416f-b363-274c96b88ae0@redhat.com>
Date:   Sat, 2 Dec 2023 17:39:36 -0500
From:   Waiman Long <longman@...hat.com>
To:     David Laight <David.Laight@...LAB.COM>,
        Jann Horn <jannh@...gle.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-kernel@...r.kernel.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>
Subject: Re: [PATCH] locking: Document that mutex_unlock() is non-atomic

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