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SubjectRe: [PATCH 1/2] Avoid bio_endio recursion
On Tuesday June 24, mpatocka@redhat.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@suse.de>

Thanks,
NeilBrown


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