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 S1753518AbdLOHcJ (ORCPT <rfc822;w@1wt.eu>);
        Fri, 15 Dec 2017 02:32:09 -0500
Received: from merlin.infradead.org ([205.233.59.134]:39240 "EHLO
        merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S1752590AbdLOHcH (ORCPT
        <rfc822;linux-kernel@vger.kernel.org>);
        Fri, 15 Dec 2017 02:32:07 -0500
Date: Fri, 15 Dec 2017 08:31:51 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: "jianchao.wang" <jianchao.w.wang@oracle.com>
Cc: Bart Van Assche <Bart.VanAssche@wdc.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-block@vger.kernel.org" <linux-block@vger.kernel.org>,
        "kernel-team@fb.com" <kernel-team@fb.com>,
        "oleg@redhat.com" <oleg@redhat.com>, "hch@lst.de" <hch@lst.de>,
        "axboe@kernel.dk" <axboe@kernel.dk>, "osandov@fb.com" <osandov@fb.com>,
        "tj@kernel.org" <tj@kernel.org>
Subject: Re: [PATCH 2/6] blk-mq: replace timeout synchronization with a RCU
 and generation based scheme
Message-ID: <20171215073151.y5nme76arcg7ra23@hirez.programming.kicks-ass.net>
References: <20171212190134.535941-1-tj@kernel.org>
 <20171212190134.535941-3-tj@kernel.org>
 <1513277469.2475.43.camel@wdc.com>
 <20171214202042.GG3326@worktop>
 <1513287766.2475.73.camel@wdc.com>
 <20171214215404.GK3326@worktop>
 <007e5a56-83fb-23b0-64d9-4725f15c596d@oracle.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <007e5a56-83fb-23b0-64d9-4725f15c596d@oracle.com>
User-Agent: NeoMutt/20170609 (1.8.3)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Fri, Dec 15, 2017 at 10:12:50AM +0800, jianchao.wang wrote:
> > That only makes it a little better:
> > 
> > 	Task-A					Worker
> > 
> > 	write_seqcount_begin()
> > 	blk_mq_rw_update_state(rq, IN_FLIGHT)
> > 	blk_add_timer(rq)
> > 	<timer>
> > 		schedule_work()
> > 	</timer>
> > 	<context-switch to worker>
> > 						read_seqcount_begin()
> > 							while(seq & 1)
> > 								cpu_relax();
> > 
> Hi Peter
> 
> The current seqcount read side is as below:
> 	do {
> 		start = read_seqcount_begin(&rq->gstate_seq);


static inline unsigned read_seqcount_begin(const seqcount_t *s)
{
	seqcount_lockdep_reader_access(s);
	return raw_read_seqcount_begin(s);
}

static inline unsigned raw_read_seqcount_begin(const seqcount_t *s)
{
	unsigned ret = __read_seqcount_begin(s);
	smp_rmb();
	return ret;
}

static inline unsigned __read_seqcount_begin(const seqcount_t *s)
{
	unsigned ret;

repeat:
	ret = READ_ONCE(s->sequence);
	if (unlikely(ret & 1)) {
		cpu_relax();
		goto repeat;
	}
	return ret;
}