linux-kernel - Re: 4.0.0-rc4: panic in free

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Date:	Sun, 22 Mar 2015 15:25:57 -0400
From:	Bob Picco <bpicco@...oft.net>
To:	David Miller <davem@...emloft.net>
Cc:	torvalds@...ux-foundation.org, david.ahern@...cle.com,
	sparclinux@...r.kernel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: 4.0.0-rc4: panic in free_block

David Miller wrote:	[Sun Mar 22 2015, 01:36:03PM EDT]
> From: Linus Torvalds <torvalds@...ux-foundation.org>
> Date: Sat, 21 Mar 2015 11:49:12 -0700
> 
> > Davem? I don't read sparc assembly, so I'm *really* not going to try
> > to verify that (a) all the memcpy implementations always copy
> > low-to-high and (b) that I even read the address comparisons in
> > memmove.S right.
> 
> All of the sparc memcpy implementations copy from low to high.
> I'll eat my hat if they don't. :-)
> 
> The guard tests at the beginning of memmove() are saying:
> 
> 	if (dst <= src)
> 		memcpy(...);
> 	if (src + len <= dst)
> 		memcpy(...);
> 
> And then the reverse copy loop (and we do have to copy in reverse for
> correctness) is basically:
> 
> 	src = (src + len - 1);
> 	dst = (dst + len - 1);
> 
> 1:	tmp = *(u8 *)src;
> 	len -= 1;
> 	src -= 1;
> 	*(u8 *)dst = tmp;
> 	dst -= 1;
> 	if (len != 0)
> 		goto 1b;
> 
> And then we return the original 'dst' pointer.
> 
> So at first glance it looks at least correct.
> 
> memmove() is a good idea to look into though, as SLAB and SLUB are the
> only really heavy users of it, and they do so with overlapping
> contents.
> 
> And they end up using that byte-at-a-time code, since SLAB and SLUB
> do mmemove() calls of the form:
> 
> 	memmove(X + N, X, LEN);
> 
> In which case neither of the memcpy() guard tests will pass.
> 
> Maybe there is some subtle bug in there I just don't see right now.
My original pursuit of this issue focused on transfers to and from the shared
array. Basically substituting memcpy-s with a primitive unsigned long memory
mover. This might have been incorrect.

There were substantial doubts because of large modifications to 2.6.39 too.
Unstabile hardware cause(d|s) issue too.

Eliminating the shared array functions correctly. Though this removal changes
performance and timing dramatically.

This afternoon I included modification of two memmove-s and no issue thus far.
The issue APPEARS to come from memmove-s within cache_flusharray() and/or
drain_array(). Now we are covering moves within an array_cache.

The above was done on 2.6.39.
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