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

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Message-ID: <4fd976bf-2a09-ef08-e6f5-039bb20d998b@redhat.com>
Date:   Sat, 2 Sep 2023 15:40:01 -0400
From:   Waiman Long <longman@...hat.com>
To:     Michał Mirosław <mirq-linux@...e.qmqm.pl>
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 9/2/23 13:06, Michał Mirosław wrote:
> 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.

Yes, I have misread the code thinking that __ww_waiter_add() with a NULL 
third argument is different from __mutex_add_waiter(). They the same in 
this case for the non-PREEMPT_RT kernel. For the PREEMPT_RT kernel, 
however, they are still different.

Cheers,
Longman