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

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Date:	Mon, 3 Mar 2014 21:26:42 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	George Spelvin <linux@...izon.com>
Cc:	torvalds@...ux-foundation.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: Update of file offset on write() etc. is non-atomic with I/O

On Mon, Mar 03, 2014 at 04:03:59PM -0500, George Spelvin 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.)

Most of the cases have it kept separately in registers, actually - there's
a reason why fdget() and friends are inlined.
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