linux-kernel - Re: [PATCH 1/3] dio: track and serialise unaligned direct IO

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Date:	Tue, 9 Nov 2010 09:55:48 +1100
From:	Dave Chinner <david@...morbit.com>
To:	Jeff Moyer <jmoyer@...hat.com>
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] dio: track and serialise unaligned direct IO

On Mon, Nov 08, 2010 at 10:28:29AM -0500, Jeff Moyer wrote:
> Dave Chinner <david@...morbit.com> writes:
> 
> > +struct dio_zero_block {
> > +	struct list_head dio_list;	/* list of io in progress */
> > +	sector_t	zero_block;	/* block being zeroed */
> > +	struct dio	*dio;		/* owner dio */
> > +	wait_queue_head_t wq;		/* New IO block here */
>                       New IOs block here, or new IO blocks here?
> 
> > +/*
> > + * Add a filesystem block to the list of blocks we are tracking.
> > + */
> > +static void
> > +dio_start_zero_block(struct dio *dio, sector_t zero_block)
> > +{
> > +	struct dio_zero_block *zb;
> > +
> > +	zb = kmalloc(sizeof(*zb), GFP_NOIO);
> > +	if (!zb)
> > +		return;
> > +	INIT_LIST_HEAD(&zb->dio_list);
> > +	init_waitqueue_head(&zb->wq);
> > +	zb->zero_block = zero_block;
> > +	zb->dio = dio;
> > +	atomic_set(&zb->ref, 1);
> > +
> > +	spin_lock(&dio_zero_block_lock);
> > +	list_add(&zb->dio_list, &dio_zero_block_list);
> > +	spin_unlock(&dio_zero_block_lock);
> 
> What protects from two processes getting here at the same time, and
> hence adding the same block to the list?  i_mutex?

The wait in dio_zero_block() called before this function is called is
supposed to serialise them, but now that you point it out, it's not
an atomic wait-and-add so ther eis a small window where two IOs
could pass through here. Easy enoug to fix by combining the search
and inѕert - I'll restructure it along those lines.

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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