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

[Al Viro <viro@ZenIV.linux.org.uk>]

On Mon, Mar 03, 2014 at 01:52:13PM -0800, Linus Torvalds wrote:
> On Mon, Mar 3, 2014 at 1:26 PM, Al Viro <viro@zeniv.linux.org.uk> wrote:
> > On Mon, Mar 03, 2014 at 04:03:59PM -0500, George Spelvin wrote:
> >>
> >> (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.
> 
> Yes. And bit test and set ops on registers are actually cheaper than
> playing around with bytes.
> 
> That said, the "fget_light()" interface sucks - exactly because it
> doesn't do the "return structure in two registers" thing. We should
> get rid of it - there's just one remaining user in networking code,
> and it should be rewritten in terms of fdget().
> 
> That's a separate issue, though.

The thing is, the callers in there do *not* keep struct file * at all -
they keep struct socket * and use sock->file to get struct file * back
when they need it.

So struct fd is the wrong thing to use there - it only adds to register
pressure.  A similar pair of struct socket * and "need to fput that"
flag would probably be needed, with sockfd_lookup_light() returning
that.