Re: [PATCH 1/2] Avoid bio_endio recursion

[Mikulas Patocka]

On Tue, 24 Jun 2008, Neil Brown wrote:

> On Tuesday June 24, mpatocka@xxxxxxxxxx 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.

Yes. I think "current" variable can't be used in irq context, it would 
blow with irq-stacks (or access some weird unknown memory).

I had another version of the patch that doesn't disable interrupts and 
only disables preempt and uses local_t atomic cpu-local variables. It is 
somehow more tricky, because interrupt can be triggered any time while 
processing the queue and it can add anything to the queue. Then I realized 
that bio_endio runs most time with disabled interrupts anyway, so it'd be 
better to just disable interrupts and don't do that local_cmpxchg 
trickery.

Mikulas

> > +
> > +	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@xxxxxxx>
>
> Thanks,
> NeilBrown
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