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

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Date:	Wed, 17 Oct 2007 14:11:34 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Jens Axboe <jens.axboe@...cle.com>
cc:	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	Jeff Garzik <jgarzik@...ox.com>,
	Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: [bug] ata subsystem related crash with latest -git



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
-
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