linux-kernel - Re: [PATCH 05/10] writeback: support

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Date:	Mon, 6 Jul 2009 14:49:30 +0100
From:	Jamie Lokier <jamie@...reable.org>
To:	Artem Bityutskiy <dedekind1@...il.com>
Cc:	Jens Axboe <jens.axboe@...cle.com>, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, chris.mason@...cle.com,
	david@...morbit.com, hch@...radead.org, akpm@...ux-foundation.org,
	jack@...e.cz, yanmin_zhang@...ux.intel.com,
	richard@....demon.co.uk, damien.wyart@...e.fr, fweisbec@...il.com,
	Alan.Brunelle@...com
Subject: Re: [PATCH 05/10] writeback: support > 1 flusher thread per bdi

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().

-- Jamie
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