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SubjectRe: [bug] block subsystem related crash with latest -git
On Thu, Oct 18 2007, David Miller wrote:
> From: Jens Axboe <>
> Date: Thu, 18 Oct 2007 12:55:17 +0200
> > Things have progressed a lot since, see my recent posting based on
> > Davem's proposal. Will post another patch soonish, that is also
> > tested.
> One core issue here is that we need to decide whether this thing to be
> iterated like an array or like a linked list. It's trying to be both.
> If we decide upon a looping construct for consumers and stick to it,
> we'll be in much better shape than we are now and bugs will be eaiser
> to spot. It would be so much simpler to audit if all we saw in the
> consumers were things like:
> while (sg) {
> do_stuff(sg);
> sg = sg_next(sg);
> }
> I would suggest that we just get it over with and convert the whole
> tree now rather than trying to do this kind of thing in stages.
> Because then we can say that ever scatterlist creator has to set
> the "end" bit and therefore you use well established patterns
> for scatterlist iteration such as "traverse sg_next() until NULL"
> as shown above.

The above should work now, PROVIDED that the end has been marked
properly. I have added some helpers to init an sglist (or sg entry).
You can still use


and pass in the number of entries, that'll work even with an end marker.
So, it does both.

> I also noticed that there is the issue of on-stack and embedded
> scatterlist users. We'll need some sort of "DECLARE_SCATTERLIST"
> and a "scatterlist_init()" thing so that we can keep DEBUG_SG
> working even in those cases. But for all I know Jens could be
> working on that already :-)

Heh, got some of it covered! The stack usage is generally just converted
to either use sg_init_one() if we can, or sg_init_table(). Adding some
soft of DECLARE_SCATTERLIST() sounds like a good idea, though. I'll take
patches :-)

> The only other real option if we don't convert the whole tree now to
> the "end" marker stuff, is to enforce that every scatterlist iterator
> only traverse the number of entries there were told are in the one
> given to them.

Driver that do all their own sg management typically still use a manual
loop over the number of entries they know are there. I didn't change
those, and the original sg chaining didn't convert those either. Some
manual labor is required in utilizing chained sg lists, unless you are
part of a subsystem (like SCSI) that allocates and inits themf or you.
So I still think that baby steps are a good idea - only convert things
that matter. Leave the rest to janitors or others willing to dive in.

Jens Axboe

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