linux-kernel - Re: Update of file offset on write() etc. is non-atomic with I/O

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Message-ID: <CAHO5Pa2tJ2XRj7R69o=0i2yV+0mmxg9uOyDQ_HFFudZ0xGmPsQ@mail.gmail.com>
Date:	Sun, 4 May 2014 09:04:00 +0200
From:	Michael Kerrisk <mtk.manpages@...il.com>
To:	George Spelvin <linux@...izon.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux-Fsdevel <linux-fsdevel@...r.kernel.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Michael Kerrisk-manpages <mtk.manpages@...il.com>
Subject: Re: Update of file offset on write() etc. is non-atomic with I/O

Ouch! I've just seen that trimming the CC on this reply took me out of
a large part of the subsequent conversation. PLEASE don't trim CCs,
and especially not the address of the OP!!

On Mon, Mar 3, 2014 at 10:03 PM, George Spelvin <linux@...izon.com> wrote:
>>  struct fd {
>>       struct file *file;
>> -     int need_put;
>> +     unsigned need_put:1, need_pos_unlock:1;
>>  };
>
> Since we're rounding up to 2*sizeof(struct file *) anyway, is this a case
> where wasting space on a couple of char (or bool) flags would generate
> better code than a bitfield?
>
> In particular, the code to set need_pos_unlock (which will be executed
> each read/write for most files) is messy in the bitfield case.
> A byte store is much cleaner.
>
> (If you want to use bits, why not use the two lsbits of the file pointer
> for the purpose?  That would save a lot of space.)
> --
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-- 
Michael Kerrisk Linux man-pages maintainer;
http://www.kernel.org/doc/man-pages/
Author of "The Linux Programming Interface", http://blog.man7.org/
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