linux-kernel - Re: [PATCH] locking/mutex: Clarify that mutex_unlock(), and most other sleeping locks, cannot be used to reference-count objects

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Date: Mon, 8 Jan 2024 16:28:11 +0100
From: Jann Horn <jannh@...gle.com>
To: Ingo Molnar <mingo@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>, linux-kernel@...r.kernel.org, 
	linux-tip-commits@...r.kernel.org, x86@...nel.org, 
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] locking/mutex: Clarify that mutex_unlock(), and most
 other sleeping locks, cannot be used to reference-count objects

On Mon, Jan 8, 2024 at 9:45 AM Ingo Molnar <mingo@...nel.org> wrote:
> * Peter Zijlstra <peterz@...radead.org> wrote:
>
> > On Fri, Dec 01, 2023 at 10:44:09AM -0000, tip-bot2 for Jann Horn wrote:
> >
> > > --- a/Documentation/locking/mutex-design.rst
> > > +++ b/Documentation/locking/mutex-design.rst
> > > @@ -101,6 +101,12 @@ features that make lock debugging easier and faster:
> > >      - Detects multi-task circular deadlocks and prints out all affected
> > >        locks and tasks (and only those tasks).
> > >
> > > +Releasing a mutex is not an atomic operation: Once a mutex release operation
> >
> > I still object to this confusing usage of atomic. Also all this also
> > applies to all sleeping locks, rwsem etc. I don't see why we need to
> > special case mutex here.
> >
> > Also completion_done() has an explicit lock+unlock on wait.lock to
> > deal with this there.
>
> Fair enough - but Jan's original observation stands: mutexes are the
> sleeping locks most similar to spinlocks, so the locking & object lifetime
> pattern that works under spinlocks cannot be carried over to mutexes in all
> cases, and it's fair to warn about this pitfall.
>
> We single out mutex_lock(), because they are the most similar in behavior
> to spinlocks, and because this concern isn't hypothethical, it has been
> observed in the wild with mutex users.
>
> How about the language in the attached patch?

In case you missed it, I sent this rewritten documentation patch in
response to the feedback I got, intended to replace the patch that is
now sitting in the tip tree (but I don't know how that works
procedurally for something that's already in the tip tree, whether
you'd want to just swap out the patch with a forced update, or revert
out the old version, or something else):
<https://lore.kernel.org/all/20231204132259.112152-1-jannh@google.com/>

Since there were comments on how this is really a more general rule
than a mutex-specific one, that version doesn't touch
Documentation/locking/mutex-design.rst and instead documents the rule
in Documentation/locking/locktypes.rst; and then it adds comments
above some of the most common unlock-type functions that would be
affected.