Re: [PATCH v2 05/11] locking/ww_mutex: Add waiters in stamp order

[Nicolai HÃhnle]

On 16.12.2016 18:15, Peter Zijlstra wrote:
> On Fri, Dec 16, 2016 at 03:19:43PM +0100, Nicolai Hähnle wrote:
> > The concern about picking up a handoff that we didn't request is real,
> > though it cannot happen in the first iteration. Perhaps this __mutex_trylock
> > can be moved to the end of the loop? See below...
>
>
> > > > @@ -728,7 +800,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
> > > > 		 * or we must see its unlock and acquire.
> > > > 		 */
> > > > 		if ((first && mutex_optimistic_spin(lock, ww_ctx, use_ww_ctx, true)) ||
> > > > -		     __mutex_trylock(lock, first))
> > > > +		     __mutex_trylock(lock, use_ww_ctx || first))
> > > > 			break;
> > > >
> > > > 		spin_lock_mutex(&lock->wait_lock, flags);
> >
> > Change this code to:
> >
> > 		acquired = first &&
> > 		    mutex_optimistic_spin(lock, ww_ctx, use_ww_ctx,
> > 					  &waiter);
> > 		spin_lock_mutex(&lock->wait_lock, flags);
> > 		
> > 		if (acquired ||
> > 		    __mutex_trylock(lock, use_ww_ctx || first))
> > 			break;
>
> 			goto acquired;
>
> will work lots better.

Wasn't explicit enough, sorry. The idea was to get rid of the acquired 
label and change things so that all paths exit the loop with wait_lock 
held. That seems cleaner to me.


> > 	}
> >
> > This changes the trylock to always be under the wait_lock, but we previously
> > had that at the beginning of the loop anyway.
>
> > It also removes back-to-back
> > calls to __mutex_trylock when going through the loop;
>
> Yeah, I had that explicitly. It allows taking the mutex when
> mutex_unlock() is still holding the wait_lock.

mutex_optimistic_spin() already calls __mutex_trylock, and for the 
no-spin case, __mutex_unlock_slowpath() only calls wake_up_q() after 
releasing the wait_lock.

So I don't see the purpose of the back-to-back __mutex_trylocks, 
especially considering that if the first one succeeds, we immediately 
take the wait_lock anyway.

Nicolai



> > and for the first
> > iteration, there is a __mutex_trylock under wait_lock already before adding
> > ourselves to the wait list.
>
> Correct.