linux-ext4 - Re: [RESEND] [PATCH] lseek: change i

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Date:	Thu, 15 Jan 2009 17:49:07 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Hisashi Hifumi <hifumi.hisashi@....ntt.co.jp>
Cc:	Theodore Tso <tytso@....edu>, matthew@....cx,
	linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [RESEND] [PATCH] lseek: change i_mutex usage.

On Fri, 16 Jan 2009 09:53:02 +0900 Hisashi Hifumi <hifumi.hisashi@....ntt.co.jp> wrote:

> 
> At 09:40 09/01/16, Andrew Morton wrote:
> >On Thu, 15 Jan 2009 09:21:13 -0500
> >Theodore Tso <tytso@....edu> wrote:
> >
> >> On Thu, Jan 15, 2009 at 06:22:52AM -0700, Matthew Wilcox wrote:
> >> > 
> >> > Of course if you have multiple threads, they will share a struct file,
> >> > and you're updating f_pos and f_version without locking.  Maybe that's
> >> > OK, but it's soemthing you didn't discuss.
> >> 
> >> f_pos is updated by sys_write(), and friends without locking, so we're
> >> fine on that front, or at least no worse off.
> >
> >bug ;)
> >
> >>  SUSv3 doesn't seem to
> >> say one way or another what should happen if two threads try to
> >> write() to a file at the same time using the same file descriptor in
> >> terms of whether or not f_pos gets updated intelligently.  We've opted
> >> for speed over determinism already.
> >
> >I think our thinking was that if two threads are racily updating f_pos
> >with different values, then it should end up with one of those values.
> >
> >From a quality-of-implementation POV (what _is_ that, anyway) it would
> >be bad if the kernel were to set f_pos to the upper 32 bits of position
> >A and the lower 32 bits of position B.  Which could happen if we remove
> >the i_mutex protection on 32-bits.
> >
> >We could perhaps omit some locking if CONFIG_64BIT.  There's probably
> >quite a bit of locking which could be omitted in that case.
> 
> Updating f_pos value on 32bit is not atomic, so we discussed about this
> but we concluded that it does not matter whether f_pos is atomic or not

It's unclear what you're saying here.

I see three issues here:

a) two racing threads update f_pos.  One of them wins, and the
   outcome in indeterminate.

b) two racing threads update f_pos and the end result is that f_pos
   contains a value which *neither* thread tried to write.

c) one thread is writing and the other reading.  There is a window
   where the reader can see an intermediate value which is a mix of the
   old and new values.

I think we decided that a) is acceptable, b) is not and that c) can only
occur on multiple-of-4G wraparounds and isn't worth bothering about.

> See,
> Subject:[RESEND] [PATCH] VFS: make file->f_pos access atomic on 32bit
> http://marc.info/?l=linux-fsdevel&m=122335627224515

Sorry, I'm disinclined to re-read a long thread, trying to work out
which bit you might be referring to.

> I think even i_mutex is not needed. When we touch i_size, i_size_read is enough,
> and we can remove i_mutex at all on lseek.

Why are we talking about i_size now?

Confused.
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