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From: Nick Piggin <nickpiggin@yahoo.com.au>
To: Andi Kleen <andi@firstfloor.org>
Subject: Re: [RESEND] [PATCH] VFS: make file->f_pos access atomic on 32bit arch
Date: Wed, 8 Oct 2008 03:27:44 +1100
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Cc: Hisashi Hifumi <hifumi.hisashi@oss.ntt.co.jp>, akpm@linux-foundation.org,
       linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org
References: <6.0.0.20.2.20081007140438.0580f110@172.19.0.2> <6.0.0.20.2.20081007183452.0f052210@172.19.0.2> <20081007102942.GE20740@one.firstfloor.org>
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On Tuesday 07 October 2008 21:29, Andi Kleen wrote:
> > Maybe cmpxchg8b is good for i486 or later x86, but i386 or other
> > architectures that do not have similar instruction needs some locking
> > primitive. I think lazy
>
> We have a cmpxchg emulation on 386. That works because only UP 386s are
> supported, so it can be done in software.
>
> > seqlock is one option for making file->f_pos access atomic.
>
> The question is if it's the right option. At least all the common
> operations on fds (read/write) are all writers, not readers.

Common operations are read, do something, write. So seqlocks then cost
one atomic operation, a couple of memory barriers (all noops on x86),
and some predictable branches etc.

cmpxchg based would require 2 lock ; cmpxchg8b on 32-bit. Fairly heavy.
Also I don't think we have generic accessors to do this, so I think
that is for another project.

Anyway, I think importantly this creates some usable accessors for the
f_pos problem. I think we actually need to touch a _lot_ of code to
cover all f_pos accesses in the kernel, but I guess this gets the ball
rolling.

So.. is everyone agreed that corrupting f_pos is a bad thing? (serious
question) If so, then we should get something like this merged sooner
rather than later.
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