Message-ID: <4A520608.7070707@gmail.com>
Date: Mon, 06 Jul 2009 17:11:20 +0300
From: Artem Bityutskiy <dedekind1@gmail.com>
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To: Jamie Lokier <jamie@shareable.org>
CC: Jens Axboe <jens.axboe@oracle.com>, linux-kernel@vger.kernel.org,
       linux-fsdevel@vger.kernel.org, chris.mason@oracle.com,
       david@fromorbit.com, hch@infradead.org, akpm@linux-foundation.org,
       jack@suse.cz, yanmin_zhang@linux.intel.com, richard@rsk.demon.co.uk,
       damien.wyart@free.fr, fweisbec@gmail.com, Alan.Brunelle@hp.com
Subject: Re: [PATCH 05/10] writeback: support > 1 flusher thread per bdi
References: <1245926523-21959-1-git-send-email-jens.axboe@oracle.com> <1245926523-21959-6-git-send-email-jens.axboe@oracle.com> <4A51FE33.3070702@gmail.com> <20090706134930.GA4987@shareable.org>
In-Reply-To: <20090706134930.GA4987@shareable.org>
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Jamie Lokier wrote:
> Artem Bityutskiy wrote:
>> Jens Axboe wrote:
>>> +static void bdi_queue_work(struct backing_dev_info *bdi, struct bdi_work 
>>> *work)
>>> +{
>>> +	if (work) {
>>> +		work->seen = bdi->wb_mask;
>>> +		BUG_ON(!work->seen);
>>> +		atomic_set(&work->pending, bdi->wb_cnt);
>>> +		BUG_ON(!bdi->wb_cnt);
>>> +
>>> +		/*
>>> +		 * Make sure stores are seen before it appears on the list
>>> +		 */
>>> +		smp_mb();
>>> +
>>> +		spin_lock(&bdi->wb_lock);
>>> +		list_add_tail_rcu(&work->list, &bdi->work_list);
>>> +		spin_unlock(&bdi->wb_lock);
>>> +	}
>> Doesn't spin_lock() include an implicit memory barrier?
>> After &bdi->wb_lock is acquired, it is guaranteed that all
>> memory operations are finished.
> 
> I'm pretty sure spin_lock() is an "acquire" barrier, which just guarantees
> loads/stores after the spin_lock() are done after taking the lock.
> 
> It doesn't guarantee anything about loads/stores before the spin_lock().

Right, but comment says memops should be flushed before the
list is changed.

-- 
Best Regards,
Artem Bityutskiy (Артём Битюцкий)
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