Re: [RFC PATCH] fs: use a sequence counter instead of file_lock in fd_install

Eric Dumazet
2015-04-20T16:46:28Z

On Mon, 2015-04-20 at 15:41 +0200, Mateusz Guzik wrote:
> On Sat, Apr 18, 2015 at 12:41:38PM -0700, Eric Dumazet wrote:
> > On Sat, 2015-04-18 at 00:02 +0100, Al Viro wrote:
> > > On Sat, Apr 18, 2015 at 12:16:48AM +0200, Mateusz Guzik wrote:
> > >
> > > > I would say this makes the use of seq counter impossible. Even if we
> > > > decided to fall back to a lock on retry, we cannot know what to do if
> > > > the slot is reserved - it very well could be that something called
> > > > close, and something else reserved the slot, so putting the file inside
> > > > could be really bad. In fact we would be putting a file for which we
> > > > don't have a reference anymore.
> > > >
> > > > However, not all hope is lost and I still think we can speed things up.
> > > >
> > > > A locking primitive which only locks stuff for current cpu and has
> > > > another mode where it locks stuff for all cpus would do the trick just
> > > > fine. I'm not a linux guy, quick search suggests 'lglock' would do what
> > > > I want.
> > > >
> > > > table reallocation is an extremely rare operation, so this should be
> > > > fine. It would take the lock 'globally' for given table.
> > >
> > > It would also mean percpu_alloc() for each descriptor table...
> >
> > I would rather use an xchg() instead of rcu_assign_ponter()
> >
> > old = xchg(&fdt->fd[fd], file);
> > if (unlikely(old))
> > filp_close(old, files);
> >
> > If threads are using close() on random fds, final result is not
> > guaranteed anyway.
> >
>
> Well I don't see how could this be used to fix the problem.
>
> If you are retrying and see NULL, you don't know whether your previous
> update was not picked up by memcpy OR the fd got closed, which also
> unreferenced the file you are installing. But you can't tell what
> happened.
>
> If you see non-NULL and what you found is not the file you are
> installing, you know the file was freed so you can't close the old file.
>
> One could try to introduce an invariant that files installed in a
> lockless manner have to start with refcount 1, you still can't infer
> anything from the fact that the counter is 1 when you retry (even if you
> take the lock). It could have been duped, or even sent over a unix
> socket and closed (although that awould surely require a solid pause in
> execution) and who knows what else.
>
> In general I would say this approach is too hard to get right to be
> worthwile given expected speedup.
>

Hey, that's because I really meant (during the week end)