linux-kernel - Re: [RFC PATCH v3 01/16] stop_machine: Fix stop_cpus_in

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Message-ID: <20190826165930.GY2369@hirez.programming.kicks-ass.net>
Date:   Mon, 26 Aug 2019 18:59:30 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     mark gross <mgross@...ux.intel.com>
Cc:     Vineeth Remanan Pillai <vpillai@...italocean.com>,
        Nishanth Aravamudan <naravamudan@...italocean.com>,
        Julien Desfossez <jdesfossez@...italocean.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>, mingo@...nel.org,
        tglx@...utronix.de, pjt@...gle.com, torvalds@...ux-foundation.org,
        linux-kernel@...r.kernel.org, subhra.mazumdar@...cle.com,
        fweisbec@...il.com, keescook@...omium.org, kerrnel@...gle.com,
        Phil Auld <pauld@...hat.com>, Aaron Lu <aaron.lwe@...il.com>,
        Aubrey Li <aubrey.intel@...il.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [RFC PATCH v3 01/16] stop_machine: Fix stop_cpus_in_progress
 ordering

On Mon, Aug 26, 2019 at 09:19:31AM -0700, mark gross wrote:
> On Wed, May 29, 2019 at 08:36:37PM +0000, Vineeth Remanan Pillai wrote:
> > From: Peter Zijlstra <peterz@...radead.org>
> > 
> > Make sure the entire for loop has stop_cpus_in_progress set.
> It is not clear how this commit comment matches the change.  Please explain
> how adding 2 barrier's makes sure stop_cpus_in_progress is set for the entier
> for loop.

Without the barrier the compiler is free to move the stores around. It
probably doesn't do anything bad, but this makes sure it cannot.