linux-kernel - Re: [PATCH] [8/18] BKL-removal: Remove BKL from remote

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Message-ID: <524f69650801271418s16f88928xc58dcbe9f5ede9e4@mail.gmail.com>
Date:	Sun, 27 Jan 2008 16:18:26 -0600
From:	"Steve French" <smfrench@...il.com>
To:	"Trond Myklebust" <Trond.Myklebust@...app.com>
Cc:	"Andi Kleen" <ak@...e.de>, swhiteho@...hat.com, sfrench@...ba.org,
	vandrove@...cvut.cz, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, akpm@...l.org
Subject: Re: [PATCH] [8/18] BKL-removal: Remove BKL from remote_llseek

If two seeks overlap, can't you end up with an f_pos value that is
different than what either thread seeked to? or if you have a seek and
a read overlap can't you end up with the read occurring in the midst
of an update of f_pos (which takes more than one instruction on
various architectures), e.g. reading an f_pos, which has only the
lower half of a 64 bit field updated?   I agree that you shouldn't
have seeks racing in parallel but I think it is preferable to get
either the updated f_pos or the earlier f_pos not something 1/2
updated.

On Jan 27, 2008 11:56 AM, Trond Myklebust <Trond.Myklebust@...app.com> wrote:
>
> On Sun, 2008-01-27 at 10:57 -0600, Steve French wrote:
> > Don't you need to a spinlock/spinunlock(i_lock) or something similar
> > (there isn't a spinlock in the file struct unfortunately) around the
> > reads and writes from f_pos in fs/read_write.c in remote_llseek with
> > your patch since the reads/writes from that field are not necessarily
> > atomic and threads could be racing in seek on the same file struct?
>
> Where does is state in POSIX or SUS that we need to cater to that kind
> of application?
> In any case, the current behaviour of f_pos if two threads are sharing
> the file struct is undefined no matter whether you spinlock or not,
> since there is no special locking around sys_read() or sys_write().
>
> Trond
>



-- 
Thanks,

Steve
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