Re: [PATCH 4/5] locking/rwsem: Avoid deceiving lock spinners

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Feb 03, 2015 at 11:43:36AM -0800, Tim Chen wrote:
> That's true.  We cannot have the lock grabbed by a new write 
> contender as any new writer contender of the lock will be 
> queued by the OSQ logic. Only the
> thread doing the optimistic spin is attempting write lock.  
> In other word, switching of write owner of the rwsem to a new
> owner cannot happen.  Either write owner stay as the original one, or
> we don't have a write owner.  So using test of write owner
> switching as an indicator of congestion is incorrect.
> 
> If my reasoning above is sound, then the check 
> 
> +       if (READ_ONCE(sem->owner))
> +               return true; /* new owner, continue spinning */
> +
> 
> is unnecessary and can be removed, as we cannot have a 
> new write owner of the rwsem, other than the thread
> doing optimistic spinning.

I have read the rest of the thread; but the one thing that I didn't see
is trylocks, trylocks can always come in an steal things regardless of
the OSQ stuff.