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 S1754371AbcGVT0S (ORCPT <rfc822;w@1wt.eu>);
	Fri, 22 Jul 2016 15:26:18 -0400
Received: from mx2.suse.de ([195.135.220.15]:48061 "EHLO mx2.suse.de"
	rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
	id S1753829AbcGVT0Q convert rfc822-to-8bit (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Fri, 22 Jul 2016 15:26:16 -0400
Date: Fri, 22 Jul 2016 12:26:06 -0700
From: Davidlohr Bueso <dave@stgolabs.net>
To: Imre Deak <imre.deak@intel.com>
Cc: Waiman Long <waiman.long@hpe.com>, Jason Low <jason.low2@hpe.com>,
        Peter Zijlstra <peterz@infradead.org>, linux-kernel@vger.kernel.org,
        Ingo Molnar <mingo@redhat.com>,
        Chris Wilson <chris@chris-wilson.co.uk>,
        Daniel Vetter <daniel.vetter@intel.com>
Subject: Re: [RFC] Avoid mutex starvation when optimistic spinning is disabled
Message-ID: <20160722192606.GC1881@linux-80c1.suse>
References: <20160718171537.GC6862@twins.programming.kicks-ass.net>
 <1468864069.2367.21.camel@j-VirtualBox>
 <1468947205.31332.40.camel@intel.com>
 <1468969470.10247.15.camel@j-VirtualBox>
 <1468989556.10247.22.camel@j-VirtualBox>
 <1469021399.19093.14.camel@intel.com>
 <1469134658.2344.12.camel@j-VirtualBox>
 <57925E25.8010506@hpe.com>
 <20160722180309.GB1881@linux-80c1.suse>
 <1469212184.9353.15.camel@intel.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=iso-8859-1; format=flowed
Content-Disposition: inline
Content-Transfer-Encoding: 8BIT
In-Reply-To: <1469212184.9353.15.camel@intel.com>
User-Agent: Mutt/1.5.24 (2015-08-30)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Fri, 22 Jul 2016, Imre Deak wrote:

>On Fri, 2016-07-22 at 11:03 -0700, Davidlohr Bueso wrote:
>> On Fri, 22 Jul 2016, Waiman Long wrote:
>>
>> > I think making mutex_trylock() fail maybe a bit too far. Do we
>> > really
>> > have any real workload that cause starvation problem  because of
>> > that.
>> > Code that does mutex_trylock() in a loop can certainly cause lock
>> > starvation, but it is not how mutex_trylock() is supposed to be
>> > used.
>> > We can't build in safeguard for all the possible abuses of the
>> > mutex
>> > APIs.
>>
>> True, and that's actually why I think that 'fixing' the
>> !SPIN_ON_OWNER case
>> is a bit too far in the first place: most of the archs that will care
>> about
>> this already have ARCH_SUPPORTS_ATOMIC_RMW. The extra code for
>> dealing with
>> this is not worth it imo.
>
>SPIN_ON_OWNER is also disabled in case of DEBUG_MUTEXES, which is the
>config where I wanted to avoid starvation in the first place.

Well yes, but know of course that that option is even less common than
archs with non atomic Rmw.

Thanks,
Davidlohr