linux-ext4 - Re: [patch] fs: revert 8ab22b9a

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Date:	Wed, 10 Sep 2008 17:47:00 +0900
From:	Hisashi Hifumi <hifumi.hisashi@....ntt.co.jp>
To:	Nick Piggin <npiggin@...e.de>,
	Christoph Hellwig <hch@...radead.org>, Jan Kara <jack@....cz>,
	linux-ext4@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [patch] fs: revert 8ab22b9a


At 13:52 08/09/10, Nick Piggin wrote:
>
>Patch 8ab22b9a, "vfs: pagecache usage optimization for pagesize!=blocksize",
>introduces a data race that might cause uninitialized data to be exposed to
>userland. The race is conceptually the same as the one fixed for page
>uptodateness, fixed by 0ed361de.
>
>The problem is that a buffer_head flags will be set uptodate after the
>stores to bring its pagecache data uptodate[*]. This patch introduces a
>possibility to read that pagecache data if the buffer_head flag has been
>found uptodate. The problem is there are no barriers or locks ordering
>the store/store vs the load/load.
>
>To illustrate:
> CPU0: write(2) (1024 bytes)           CPU1: read(2) (1024 bytes)
> 1. allocate new pagecache page        A. locate page, not fully uptodate
> 2. copy_from_user to part of page     B. partially uptodate? load bh flags
> 3. mark that buffer uptodate          C. if yes, then copy_to_user
>
>So if the store 3 is allowed to execute before the store 2, and/or the
>load in C is allowed to execute before the load in B, then we can wind
>up loading !uptodate data.
>

>
>One way to solve this is to add barriers to the buffer head operations
>similarly to the fix for the page issue. The problem is that, unlike the
>page race, we don't actually *need* to do that if we decide not to support
>this functionality. The barriers are quite heavyweight on some
>architectures, and we haven't seen really compelling numbers in favour of
>this patch yet (a best-case microbenchmark showed some improvement of
>course, but with memory barriers we could also produce a worst-case bench
>that shows some slowdown on many architectures).

I think that adding wmb/rmb to all buffer_uptodate/set_buffer_uptodate is heavy
on some architectures using BUFFER_FNS macros, but it can be possible
to mitigate performance slowdown by minimizing memory barrier utilization.
The patch "vfs: pagecache usage optimization for pagesize!=blocksize" is now 
just for ext2/3/4, so is it not sufficient to solve the above uninitialized data
exposure problem that adding one rmb to block_is_partially_uptodate() 
and wmb to __block_commit_write() ?

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