linux-kernel - Re: [PATCH 4/9] firewire: don't use PREPARE_DELAYED

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Message-ID: <5306B4DF.4000901@hurleysoftware.com>
Date:	Thu, 20 Feb 2014 21:07:27 -0500
From:	Peter Hurley <peter@...leysoftware.com>
To:	Tejun Heo <tj@...nel.org>
CC:	laijs@...fujitsu.com, linux-kernel@...r.kernel.org,
	Stefan Richter <stefanr@...6.in-berlin.de>,
	linux1394-devel@...ts.sourceforge.net,
	Chris Boot <bootc@...tc.net>, linux-scsi@...r.kernel.org,
	target-devel@...r.kernel.org
Subject: Re: [PATCH 4/9] firewire: don't use PREPARE_DELAYED_WORK

On 02/20/2014 08:59 PM, Tejun Heo wrote:
> Hello,
>
> On Thu, Feb 20, 2014 at 08:44:46PM -0500, Peter Hurley wrote:
>>> +static void fw_device_workfn(struct work_struct *work)
>>> +{
>>> +	struct fw_device *device = container_of(to_delayed_work(work),
>>> +						struct fw_device, work);
>>
>> I think this needs an smp_rmb() here.
>
> The patch is equivalent transformation and the whole thing is
> guaranteed to have gone through pool->lock.  No explicit rmb
> necessary.

The spin_unlock_irq(&pool->lock) only guarantees completion of
memory operations _before_ the unlock; memory operations which occur
_after_ the unlock may be speculated before the unlock.

IOW, unlock is not a memory barrier for operations that occur after.

>> IOW, the beginning of the work function should act like a barrier in
>> the same way that queue_work_on() (et. al.) already does.
>
> workqueue already has enough barriers; otherwise, the whole kernel
> would have crumbled long time ago.

See above.

Regards,
Peter Hurley
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