Re: [bug] ata subsystem related crash with latest -git

FUJITA Tomonori
2007-10-18T00:22:39Z

On Wed, 17 Oct 2007 14:11:34 -0700 (PDT)
Linus Torvalds <torvalds@linux-foundation.org> wrote:

>
>
> On Wed, 17 Oct 2007, Jens Axboe wrote:
> >
> > That would hurt... Care to commit your for_each_sg() uglification fixup
> > for now then? Or disable the allocation debug config entry, so that the
> > sg+1 deref wont crash?
>
> Well, in practice, it will only crash with DEBUG_PAGEALLOC, so few enough
> are going to be hit by it. In that sense I don't think we're in any deep
> trouble yet.
>
> That said, maybe this is an acceptable, if hacky, replacement for the
> current "for_each_sg()" loop.
>
> It does:
> - starts at one *before* the sglist
> - does the sg_next() at the *top* of the loop rather than the bottom of it
> - has a "--count" before that sg_next, so that we don't do it for the
> case when we break out and have used up all segments.
>
> Totally untested, but it *may* work, and it doesn't look horribly ugly.
>
> Ingo, does this actually make any difference?
>
> Linus
>
> ---
> include/linux/scatterlist.h | 9 ++++++++-
> 1 files changed, 8 insertions(+), 1 deletions(-)
>
> diff --git a/include/linux/scatterlist.h b/include/linux/scatterlist.h
> index 2dc7464..f5c8e11 100644
> --- a/include/linux/scatterlist.h
> +++ b/include/linux/scatterlist.h
> @@ -51,11 +51,18 @@ static inline struct scatterlist *sg_next(struct scatterlist *sg)
> return sg;
> }
>
> +static inline struct scatterlist *sg_safe_next(struct scatterlist *sg, int left)
> +{
> + if (left < 0)
> + return NULL;
> + return sg_next(sg);
> +}
> +
> /*
> * Loop over each sg element, following the pointer to a new list if necessary
> */
> #define for_each_sg(sglist, sg, nr, __i) \
> - for (__i = 0, sg = (sglist); __i < (nr); __i++, sg = sg_next(sg))
> + for (__i = (nr), sg = (sglist)-1; (sg = sg_safe_next(sg, --__i)) != NULL ; )

Looks that (sglist) - 1 isn't initialized and we use sg_next for it?