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    SubjectRe: [bug] block subsystem related crash with latest -git
    On Thu, Oct 18 2007, David Miller wrote:
    > From: Jens Axboe <jens.axboe@oracle.com>
    > 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

    for_each_sg()

    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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