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

Eric Dumazet
2015-04-22T13:55:20Z

On Wed, 2015-04-22 at 15:31 +0200, Mateusz Guzik wrote:
> On Tue, Apr 21, 2015 at 02:06:53PM -0700, Eric Dumazet wrote:
> > On Tue, 2015-04-21 at 22:12 +0200, Mateusz Guzik wrote:
> >
> > > in dup_fd:
> > > for (i = open_files; i != 0; i--) {
> > > struct file *f = *old_fds++;
> > > if (f) {
> > > get_file(f);
> > >
> >
> > I see no new requirement here. f is either NULL or not.
> > multi threaded programs never had a guarantee dup_fd() would catch a non
> > NULL pointer here.
> >
>
> It's not about seeing NULL f or not, but using the right address for
> dereference.
>
> If I read memory-barriers.txt right (see 'DATA DEPENDENCY BARRIERS'), it
> is possible that cpus like alpha will see a non-NULL pointer and then
> proceed to dereference *the old* (here: NULL) value.
>
> Hence I suspect this needs smp_read_barrier_depends (along with
> ACCESS_ONCE).
>
> Other consumers (e.g. procfs code) use rcu_dereference macro which does
> ends up using lockless_dereference macro, which in turn does:
> #define lockless_dereference(p) \
> ({ \
> typeof(p) _________p1 = ACCESS_ONCE(p); \
> smp_read_barrier_depends(); /* Dependency order vs. p
> above. */ \
> (_________p1); \
> })
>
> That said memory barriers are not exactly my strong suit, but I do
> believe my suspicion here is justified enough to ask someone with solid
> memory barrier-fu to comment.

Again, your comment has nothing to do with the patch.

If there is old data, it only can be a NULL. And it is fine, case was
_already_ handled.

It can not be an 'old' file pointer, because close() takes the spinlock.
spin_unlock() contains a write memory barrier, so the NULL pointer put
by close() would have been committed to memory.

This works also on alpha cpus.