linux-kernel - Re: [PATCH 1/3] relay: Fix 4 off-by-one errors occuring when writing to a CPU buffer.

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Message-ID: <20080614175223.4d3c6084@linux360.ro>
Date:	Sat, 14 Jun 2008 17:52:23 +0300
From:	Eduard - Gabriel Munteanu <eduard.munteanu@...ux360.ro>
To:	Tom Zanussi <tzanussi@...il.com>
Cc:	penberg@...helsinki.fi, akpm@...ux-foundation.org,
	compudj@...stal.dyndns.org, linux-kernel@...r.kernel.org,
	righi.andrea@...il.com
Subject: Re: [PATCH 1/3] relay: Fix 4 off-by-one errors occuring when
 writing to a CPU buffer.

On Fri, 13 Jun 2008 23:40:37 -0500
Tom Zanussi <tzanussi@...il.com> wrote:

> I'm wondering if the all-zeroes at the end of the buffer might be
> another case of the all-zeroes you were seeing due to cross-cpu
> reading you decribed in the other patch.  In any case, I'm pretty
> sure this patch isn't doing what you think it is, and don't see how
> it could have fixed the problem (see below).  There may still be a
> bug somewhere, but it would be good to be able to reproduce it.  Does
> it happen even when running on a single cpu?

Hi,

I noticed this problem after adding those spinlocks. As far as I can
tell, having (offset == subbuf_size + 1) at any given moment allows the
read() handler to see inconsistent offsets:
1. writer sets offset = subbuf_size + 1
2. writer releases spinlock
3. read() acquires spinlock and reads the wrong offset
4. read() releases spinlock
5. next writer corrects the offset at the next write

> This case, offset being 1 larger than the subbuf size, is how we note
> a full sub-buffer, so changing this will break full-subbuffer cases. 

No, it won't. Maximum length messages result in the following condition:
start + offset == subbuf_size
This happens because a buffer of length subbuf_size actually ranges
from zero to (subbuf_size - 1) in regard to how it is addressed. Then,
subbuf_size + 1 isn't just outside the bounds, but one more byte off.
"Visual" example:
subbuf_size = 4
|[ ][ ][ ][ ]|[ ]
  0  1  2  3   subbuf_size

So, a full subbufer means offset equals subbuf_size, that is, the next
empty slot is just outside the subbuffer.

	Eduard
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