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

[Al Viro]

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.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/