linux-kernel - Re: [RFC PATCH] fs: use a sequence counter instead of file_lock in fd

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Date:	Wed, 22 Apr 2015 15:31:51 +0200
From:	Mateusz Guzik <mguzik@...hat.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Al Viro <viro@...IV.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Yann Droneaud <ydroneaud@...eya.com>,
	Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] fs: use a sequence counter instead of file_lock in
 fd_install

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
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