Re: [PATCH] locking/mutex: remove redundant argument from __mutex_lock_common()

[Waiman Long]

On 9/2/23 15:40, Waiman Long wrote:
> 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.

OTOH, the rtmutex code will not call __ww_mutex_add_waiter() with NULL 
ww_ctx. So in that sense, the patch is probably OK. You will need to 
expand the patch description to also describe the case for PREEMPT_RT 
kernel.

Cheers,
Longman