linux-kernel - Re: [PATCH] workqueue: skip nr_running sanity check in worker_enter

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Date:	Mon, 14 May 2012 15:41:23 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] workqueue: skip nr_running sanity check in
 worker_enter_idle() if trustee is active

On Mon, May 14, 2012 at 03:12:50PM -0700, Tejun Heo wrote:
> >From 544ecf310f0e7f51fa057ac2a295fc1b3b35a9d3 Mon Sep 17 00:00:00 2001
> From: Tejun Heo <tj@...nel.org>
> Date: Mon, 14 May 2012 15:04:50 -0700
> 
> worker_enter_idle() has WARN_ON_ONCE() which triggers if nr_running
> isn't zero when every worker is idle.  This can trigger spuriously
> while a cpu is going down due to the way trustee sets %WORKER_ROGUE
> and zaps nr_running.
> 
> It first sets %WORKER_ROGUE on all workers without updating
> nr_running, releases gcwq->lock, schedules, regrabs gcwq->lock and
> then zaps nr_running.  If the last running worker enters idle
> inbetween, it would see stale nr_running which hasn't been zapped yet
> and trigger the WARN_ON_ONCE().
> 
> Fix it by performing the sanity check iff the trustee is idle.
> 
> Signed-off-by: Tejun Heo <tj@...nel.org>
> Reported-by: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> Cc: stable@...r.kernel.org
> ---
> Sorry about the delay.  After scratching my head quite a bit, I found
> where during cpu-offlining such discrepancy may happen.  I'm fairly
> sure this is it but I might be wrong, so please include this patch in
> your test setup and let me know how it goes.

Thank you -- I have applied it, and will let you know how it goes.

							Thanx, Paul

> Thank you.
> 
>  kernel/workqueue.c |    9 +++++++--
>  1 files changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/workqueue.c b/kernel/workqueue.c
> index 211eadb..c36c86c 100644
> --- a/kernel/workqueue.c
> +++ b/kernel/workqueue.c
> @@ -1213,8 +1213,13 @@ static void worker_enter_idle(struct worker *worker)
>  	} else
>  		wake_up_all(&gcwq->trustee_wait);
> 
> -	/* sanity check nr_running */
> -	WARN_ON_ONCE(gcwq->nr_workers == gcwq->nr_idle &&
> +	/*
> +	 * Sanity check nr_running.  Because trustee releases gcwq->lock
> +	 * between setting %WORKER_ROGUE and zapping nr_running, the
> +	 * warning may trigger spuriously.  Check iff trustee is idle.
> +	 */
> +	WARN_ON_ONCE(gcwq->trustee_state == TRUSTEE_DONE &&
> +		     gcwq->nr_workers == gcwq->nr_idle &&
>  		     atomic_read(get_gcwq_nr_running(gcwq->cpu)));
>  }
> 
> -- 
> 1.7.7.3
> 

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