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

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Message-ID: <20190826161929.GB2680@u1904>
Date:   Mon, 26 Aug 2019 09:19:31 -0700
From:   mark gross <mgross@...ux.intel.com>
To:     Vineeth Remanan Pillai <vpillai@...italocean.com>
Cc:     Nishanth Aravamudan <naravamudan@...italocean.com>,
        Julien Desfossez <jdesfossez@...italocean.com>,
        Peter Zijlstra <peterz@...radead.org>,
        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 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.

--mark

> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
>  kernel/stop_machine.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
> index 067cb83f37ea..583119e0c51c 100644
> --- a/kernel/stop_machine.c
> +++ b/kernel/stop_machine.c
> @@ -375,6 +375,7 @@ static bool queue_stop_cpus_work(const struct cpumask *cpumask,
>  	 */
>  	preempt_disable();
>  	stop_cpus_in_progress = true;
> +	barrier();
>  	for_each_cpu(cpu, cpumask) {
>  		work = &per_cpu(cpu_stopper.stop_work, cpu);
>  		work->fn = fn;
> @@ -383,6 +384,7 @@ static bool queue_stop_cpus_work(const struct cpumask *cpumask,
>  		if (cpu_stop_queue_work(cpu, work))
>  			queued = true;
>  	}
> +	barrier();
>  	stop_cpus_in_progress = false;
>  	preempt_enable();
>  
> -- 
> 2.17.1
>