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

Linus Torvalds
2007-10-17T21:13:23Z

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 ; )

/**
* sg_last - return the last scatterlist entry in a list