Re: [PATCH] workqueue: UNBOUND -> REBIND morphing in rebind_workers() should be atomic

From: Lai Jiangshan <laijs@cn.fujitsu.com>
To: Tejun Heo <tj@kernel.org>
Cc: linux-kernel@vger.kernel.org, stable@vger.kernel.org
Subject: Re: [PATCH] workqueue: UNBOUND -> REBIND morphing in rebind_workers() should be atomic
Date: Wed, 05 Sep 2012 09:05:37 +0800	[thread overview]
Message-ID: <5046A561.9050903@cn.fujitsu.com> (raw)
In-Reply-To: <20120904233901.GC9092@dhcp-172-17-108-109.mtv.corp.google.com>

On 09/05/2012 07:39 AM, Tejun Heo wrote:
>>From d2ae38fc5e37b4bca3c4bec04a10dcf861a77b2b Mon Sep 17 00:00:00 2001
> From: Lai Jiangshan <laijs@cn.fujitsu.com>
> Date: Sun, 2 Sep 2012 00:28:19 +0800
> 
> The compiler may compile the following code into TWO write/modify
> instructions.
> 
> 	worker->flags &= ~WORKER_UNBOUND;
> 	worker->flags |= WORKER_REBIND;
> 
> so the other CPU may temporarily see worker->flags which doesn't have
> either WORKER_UNBOUND or WORKER_REBIND set and perform local wakeup
> prematurely.
> 
> Fix it by using single explicit assignment via ACCESS_ONCE().
> 
> Because idle workers have another WORKER_NOT_RUNNING flag, this bug
> doesn't exist for them; however, update it to use the same pattern for
> consistency.
> 
> tj: Applied the change to idle workers too and updated comments and
>     patch description a bit.

Hi, tj

Thank you for accepting this one.

I'm waiting for your comments on the other patches.
I need to rebase the other patches(on top of wq/for-3.7 and this merged one),
but I think it is not good to seed to new version patchset without considering
your comments. so I'm still waiting. Or should I rebase them at first?

Thanks
Lai

> 
> Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
> Signed-off-by: Tejun Heo <tj@kernel.org>
> Cc: stable@vger.kernel.org
> ---
> Applied to wq/for-3.6-fixes with some modifications.  Greg, this won't
> apply cleanly to -stable.  Will post the backported version as a reply
> to this message.
> 
> Thanks!
> 
>  kernel/workqueue.c |   17 +++++++++++------
>  1 files changed, 11 insertions(+), 6 deletions(-)
> 
> diff --git a/kernel/workqueue.c b/kernel/workqueue.c
> index 692d976..c462cd6 100644
> --- a/kernel/workqueue.c
> +++ b/kernel/workqueue.c
> @@ -1396,12 +1396,15 @@ retry:
>  	/* set REBIND and kick idle ones, we'll wait for these later */
>  	for_each_worker_pool(pool, gcwq) {
>  		list_for_each_entry(worker, &pool->idle_list, entry) {
> +			unsigned long worker_flags = worker->flags;
> +
>  			if (worker->flags & WORKER_REBIND)
>  				continue;
>  
> -			/* morph UNBOUND to REBIND */
> -			worker->flags &= ~WORKER_UNBOUND;
> -			worker->flags |= WORKER_REBIND;
> +			/* morph UNBOUND to REBIND atomically */
> +			worker_flags &= ~WORKER_UNBOUND;
> +			worker_flags |= WORKER_REBIND;
> +			ACCESS_ONCE(worker->flags) = worker_flags;
>  
>  			idle_rebind.cnt++;
>  			worker->idle_rebind = &idle_rebind;
> @@ -1434,10 +1437,12 @@ retry:
>  	/* rebind busy workers */
>  	for_each_busy_worker(worker, i, pos, gcwq) {
>  		struct work_struct *rebind_work = &worker->rebind_work;
> +		unsigned long worker_flags = worker->flags;
>  
> -		/* morph UNBOUND to REBIND */
> -		worker->flags &= ~WORKER_UNBOUND;
> -		worker->flags |= WORKER_REBIND;
> +		/* morph UNBOUND to REBIND atomically */
> +		worker_flags &= ~WORKER_UNBOUND;
> +		worker_flags |= WORKER_REBIND;
> +		ACCESS_ONCE(worker->flags) = worker_flags;
>  
>  		if (test_and_set_bit(WORK_STRUCT_PENDING_BIT,
>  				     work_data_bits(rebind_work)))