linux-kernel - Re: [PATCH 04/11] writeback: switch to per-bdi threads for flushing data

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Message-ID: <20090527112412.GZ11363@kernel.dk>
Date:	Wed, 27 May 2009 13:24:12 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	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
Subject: Re: [PATCH 04/11] writeback: switch to per-bdi threads for
	flushing data

On Wed, May 27 2009, Peter Zijlstra wrote:
> On Wed, 2009-05-27 at 11:41 +0200, Jens Axboe wrote:
> 
> > +	if (writeback_acquire(bdi)) {
> > +		bdi->wb_arg.nr_pages = nr_pages;
> > +		bdi->wb_arg.sb = sb;
> > +		bdi->wb_arg.sync_mode = sync_mode;
> > +		/*
> > +		 * make above store seen before the task is woken
> > +		 */
> > +		smp_mb();
> > +		wake_up(&bdi->wait);
> > +	}
> 
> wake_up() implies a wmb() when we indeed to a wakeup, is that
> sufficient?

That is sufficient. I'll kill it in the next revision, seeing as this is
just an intermediate step, no harm done.

> > +int bdi_writeback_task(struct backing_dev_info *bdi)
> > +{
> > +	while (!kthread_should_stop()) {
> > +		unsigned long wait_jiffies;
> > +		DEFINE_WAIT(wait);
> > +
> > +		prepare_to_wait(&bdi->wait, &wait, TASK_INTERRUPTIBLE);
> > +		wait_jiffies = msecs_to_jiffies(dirty_writeback_interval * 10);
> > +		schedule_timeout(wait_jiffies);
> > +		try_to_freeze();
> > +
> > +		/*
> > +		 * We get here in two cases:
> > +		 *
> > +		 *  schedule_timeout() returned because the dirty writeback
> > +		 *  interval has elapsed. If that happens, we will be able
> > +		 *  to acquire the writeback lock and will proceed to do
> > +		 *  kupdated style writeout.
> > +		 *
> > +		 *  Someone called bdi_start_writeback(), which will acquire
> > +		 *  the writeback lock. This means our writeback_acquire()
> > +		 *  below will fail and we call into bdi_pdflush() for
> > +		 *  pdflush style writeout.
> > +		 *
> > +		 */
> > +		if (writeback_acquire(bdi))
> > +			bdi_kupdated(bdi);
> > +		else
> > +			bdi_pdflush(bdi);
> > +
> > +		writeback_release(bdi);
> > +		finish_wait(&bdi->wait, &wait);
> > +	}
> > +
> > +	return 0;
> > +}
> 
> the unpaired writeback_release() wrt writeback_acquire() looks odd.

Did you read the comment? :-)

> Also the prepare/finish wait bits seem oddly out of place. Are there
> really multiple waiters on bdi->wait? The above wake_up() seems to
> suggest not, since it directly modifies bdi state instead of queueing
> work.

Intermediate step, further along it should be more clear.

-- 
Jens Axboe

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