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

[Peter Zijlstra <peterz@infradead.org>]

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.

> 	}
> 
> 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.

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

Correct.