linux-kernel - Re: [PATCH] locking/mutex: remove redundant argument from __mutex_lock

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Date:   Sat, 2 Sep 2023 19:06:49 +0200
From:   Michał Mirosław <mirq-linux@...e.qmqm.pl>
To:     Waiman Long <longman@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] locking/mutex: remove redundant argument from
 __mutex_lock_common()

On Wed, Aug 30, 2023 at 07:54:13PM -0400, Waiman Long wrote:
> On 8/30/23 18:12, Michał Mirosław wrote:
> > use_ww_ctx is equivalent to ww_ctx != NULL. The one case where
> > use_ww_ctx was true but ww_ctx == NULL leads to the same
> > __mutex_add_waiter() call via __ww_mutex_add_waiter().
> I think ww_mutex_lock() can be called with a NULL ctx. Your patch will
> effectively change those ww_mutex_lock() to be equivalent to mutex_lock().
> So it is a behavioral change.

Isn't ww_mutex_lock() with ctx = NULL expected to behave like mutex_lock()?

> > Since now __ww_mutex_add_waiter() is called only with ww_mutex != NULL,
> > remove the branch there.
[...]
> > @@ -627,12 +624,11 @@ __mutex_lock_common(struct mutex *lock, unsigned int state, unsigned int subclas
> >   	debug_mutex_lock_common(lock, &waiter);
> >   	waiter.task = current;
> > -	if (use_ww_ctx)
> > -		waiter.ww_ctx = ww_ctx;
> > +	waiter.ww_ctx = ww_ctx;
> This one is fine.
> >   	lock_contended(&lock->dep_map, ip);
> > -	if (!use_ww_ctx) {
> > +	if (!ww_ctx) {
> That change will break ww_mutex.

I see that there is the rt_mutex version that stubs out
__ww_mutex_add_waiter(), but its ww_mutex_lock() doesn't use
__mutex_lock_common() at all. With the RT version out of the picture, we
can see that __ww_mutex_add_waiter(), when passed ww_ctx == NULL, just
forwards the work to __ww_waiter_add() with the same arguments
and returns 0 -- making the path exactly as the !use_ww_ctx branch.

Note: There is a lot of templating-via-preprocessor code here and I
might have missed something. I'll appreciate hints here as maybe it
could be made simpler or better understood.

Best Regards
Michał Mirosław