linux-kernel - Re: [PATCH 1/2] Avoid bio

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Message-ID: <18528.36687.631672.462076@notabene.brown>
Date:	Tue, 24 Jun 2008 16:08:15 +1000
From:	Neil Brown <neilb@...e.de>
To:	Mikulas Patocka <mpatocka@...hat.com>
Cc:	linux-kernel@...r.kernel.org, axboe@...nel.dk
Subject: Re: [PATCH 1/2] Avoid bio_endio recursion

On Tuesday June 24, mpatocka@...hat.com wrote:
> Hi
> 
> bio_endio calls bi_end_io callback. In case of stacked devices (raid, dm), 
> bio_end_io may call bio_endio again, up to an unspecified length.
> 
> The crash because of stack overflow was really observed on sparc64. And 
> this recursion was one of the contributing factors (using 9 stack frames 
> --- that is 1728 bytes).
> 
> This patch removes the recursion.

This is very cool, thanks!  A close mirror of the recursion
avoidance in generic_make_request.

You use a per-cpu queue were generic_make_request uses a per-task
queue.  This is fitting as bi_end_io doesn't have a process context,
but is supposed to be fast and able to run with interrupts disabled,
so tying to a cpu is no problem.

> +
> +	bio_queue = NULL;
> +queue_empty_next_bio:
> +	*bio_end_queue_ptr = &bio_queue;
> +next_bio:
> +
>   	if (error)
>   		clear_bit(BIO_UPTODATE, &bio->bi_flags);
>   	else if (!test_bit(BIO_UPTODATE, &bio->bi_flags))
> @@ -1175,6 +1196,17 @@
> 
>   	if (bio->bi_end_io)
>   		bio->bi_end_io(bio, error);
> +
> +	if (bio_queue) {
> +		bio = bio_queue;
> +		bio_queue = bio->bi_next;
> +		if (!bio_queue) goto queue_empty_next_bio;
> +		goto next_bio;

checkpatch.pl doesn't like that:
ERROR: trailing statements should be on next line

and I don't either.  I would not bother with the mini-optimisation at
all.
Discard the queue_empty_next_bio label and replace the "if () goto"
with
       if (!bio_queue)
		*bio_end_queue_ptr = &bio_queue;

and leave gcc to optimise the assignment if it wants to.

Reviewed-by: NeilBrown <neilb@...e.de>

Thanks,
NeilBrown
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