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

Mateusz Guzik
2015-04-22T13:32:20Z

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.

>
> > at least a data dependency barrier, or maybe smp_rmb for peace of mind
> >
> > similarly in do_dup2:
> > tofree = fdt->fd[fd];
> > if (!tofree && fd_is_open(fd, fdt))
> > goto Ebusy;
>
> Same here.
>
>

--
Mateusz Guzik