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

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Date:	Wed, 17 Oct 2007 21:35:43 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
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, Oct 17 2007, Linus Torvalds wrote:
> 
> 
> On Wed, 17 Oct 2007, Ingo Molnar wrote:
> > 
> > nope, this did not help. First bootup went fine, second bootup crashed 
> > again (see below), without hitting the BUG_ON().
> 
> I think you'll always hit it if you have a scatter-gather list that is 
> exactly filled up.
> 
> Why? Because those things do "sg_next()" on the last entry, and as 
> mentioned, that ends up actually accessing one past the end - even if the 
> end result is not actually ever *used* (because we just effectively 
> incremented it to past the last entry when the code was done with the SG 
> list).
> 
> So I think the sg_next() interface is fundamentally mis-designed. It 
> should do the scatter-gather link following on *starting* to use the SG 
> entry, not after finishing with it.
> 
> Put another way: I suspect pretty much every single sg_next() out there is 
> likely to hit this issue. The way that blk_rq_map_sg() fixed its problem 
> was exactly to move the "sg_next()" to *before* the use of the SG (and 
> even that one is somewhat bogus, in that it just blindly assumes that the 
> first entry is not a link entry).
> 
> I suspect the "the next entry is a link" bit should be in the *previous* 
> entry, and then sg_next() could look like
> 
> 	if (next_is_link(sg))
> 		sg = sg_chain_ptr(sg+1);
> 	else
> 		sg++;
> 	return sg;
> 
> and that would work. 
> 
> The alternative is to always make sure to allocate one more SG entry than 
> required, so that the last entry is always either the link, or an unused 
> entry!

OK, I think you have a very good point here. Ingo, can you verify it
goes away if apply the below? I have to tend to Other Life stuff but
will take this up again tomorrow morning and fix the sg_next() usage.

Linus, please still pull the branch I asked you to earlier. Thanks!


diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
index aac8a02..58ede7e 100644
--- a/drivers/scsi/scsi_lib.c
+++ b/drivers/scsi/scsi_lib.c
@@ -39,7 +39,7 @@
  * (unless chaining is used). Should ideally fit inside a single page, to
  * avoid a higher order allocation.
  */
-#define SCSI_MAX_SG_SEGMENTS	128
+#define SCSI_MAX_SG_SEGMENTS	129
 
 struct scsi_host_sg_pool {
 	size_t		size;


-- 
Jens Axboe

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