linux-kernel - Re: [2.6.22 PATCH 22/26] dm: bio list helpers

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Message-ID: <20070509064923.GO4163@kernel.dk>
Date:	Wed, 9 May 2007 08:49:23 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Alasdair G Kergon <agk@...hat.com>, dm-devel@...hat.com,
	linux-kernel@...r.kernel.org, Heinz Mauelshagen <hjm@...hat.com>
Subject: Re: [2.6.22 PATCH 22/26] dm: bio list helpers

On Tue, May 08 2007, Andrew Morton wrote:
> >  static inline void bio_list_init(struct bio_list *bl)
> >  {
> >  	bl->head = bl->tail = NULL;
> >  }
> >  
> > +#define bio_list_for_each(bio, bl) \
> > +	for (bio = (bl)->head; bio && ({ prefetch(bio->bi_next); 1; }); \
> > +	     bio = bio->bi_next)
> > +
> 
> I have this vague memory from a long time ago that one particular CPU type
> wants to go oops when prefetching from an invalid address.
> 
> Maybe that went away for some reason - we'd have hit it again if it was
> real.

Besides, manual prefetching is very rarely a win. I dabbled with some
benchmarks a few weeks back (with the doubly linked lists), and in most
cases it was actually a loss. So I'd vote for just removing the
prefetch() above.

-- 
Jens Axboe

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