From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S932224AbcEKSiQ (ORCPT <rfc822;w@1wt.eu>);
	Wed, 11 May 2016 14:38:16 -0400
Received: from bombadil.infradead.org ([198.137.202.9]:41876 "EHLO
	bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1751836AbcEKSiP (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Wed, 11 May 2016 14:38:15 -0400
Date: Wed, 11 May 2016 20:38:09 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Jason Low <jason.low2@hp.com>
Cc: Ingo Molnar <mingo@redhat.com>, linux-kernel@vger.kernel.org,
        Davidlohr Bueso <dave@stgolabs.net>,
        Scott J Norton <scott.norton@hpe.com>,
        Waiman Long <Waiman.Long@hpe.com>, peter@hurleysoftware.com,
        jason.low2@hpe.com
Subject: Re: [PATCH] locking/rwsem: Optimize write lock slowpath
Message-ID: <20160511183809.GJ3192@twins.programming.kicks-ass.net>
References: <1462821397.2701.16.camel@j-VirtualBox>
 <20160511114918.GG3190@twins.programming.kicks-ass.net>
 <1462991162.2488.23.camel@j-VirtualBox>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <1462991162.2488.23.camel@j-VirtualBox>
User-Agent: Mutt/1.5.21 (2012-12-30)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Wed, May 11, 2016 at 11:26:02AM -0700, Jason Low wrote:
> On Wed, 2016-05-11 at 13:49 +0200, Peter Zijlstra wrote:

> > >  static inline bool rwsem_try_write_lock(long count, struct rw_semaphore *sem)
> > >  {
> > >  	/*
> > > +	 * Avoid trying to acquire write lock if count isn't RWSEM_WAITING_BIAS.
> > >  	 */
> > > +	if (count != RWSEM_WAITING_BIAS)
> > > +		return false;
> > > +
> > > +	/*
> > > +	 * Acquire the lock by trying to set it to ACTIVE_WRITE_BIAS. If there
> > > +	 * are other tasks on the wait list, we need to add on WAITING_BIAS.
> > > +	 */
> > > +	count = list_is_singular(&sem->wait_list) ?
> > > +			RWSEM_ACTIVE_WRITE_BIAS :
> > > +			RWSEM_ACTIVE_WRITE_BIAS + RWSEM_WAITING_BIAS;
> > > +
> > > +	if (cmpxchg_acquire(&sem->count, RWSEM_WAITING_BIAS, count) == RWSEM_WAITING_BIAS) {
> > >  		rwsem_set_owner(sem);
> > >  		return true;
> > >  	}
> > 
> > Right; so that whole thing works because we're holding sem->wait_lock.
> > Should we clarify that someplace?
> 
> Yup, we can mention that the rwsem_try_write_lock() function must be
> called with the wait_lock held.

Also try to explain _why_ it must be held.

Thanks!