Re: [PATCH v2] sunrpc: fix waitqueue_active without memory barrier in sunrpc

["J. Bruce Fields" <bfields@fieldses.org>]

On Mon, Oct 12, 2015 at 10:41:06AM +0000, Kosuke Tatsukawa wrote:
> J. Bruce Fields wrote:
> > On Fri, Oct 09, 2015 at 06:29:44AM +0000, Kosuke Tatsukawa wrote:
> >> Neil Brown wrote:
> >> > Kosuke Tatsukawa <tatsu@ab.jp.nec.com> writes:
> >> > 
> >> >> There are several places in net/sunrpc/svcsock.c which calls
> >> >> waitqueue_active() without calling a memory barrier.  Add a memory
> >> >> barrier just as in wq_has_sleeper().
> >> >>
> >> >> I found this issue when I was looking through the linux source code
> >> >> for places calling waitqueue_active() before wake_up*(), but without
> >> >> preceding memory barriers, after sending a patch to fix a similar
> >> >> issue in drivers/tty/n_tty.c  (Details about the original issue can be
> >> >> found here: https://lkml.org/lkml/2015/9/28/849).
> >> > 
> >> > hi,
> >> > this feels like the wrong approach to the problem.  It requires extra
> >> > 'smb_mb's to be spread around which are hard to understand as easy to
> >> > forget.
> >> > 
> >> > A quick look seems to suggest that (nearly) every waitqueue_active()
> >> > will need an smb_mb.  Could we just put the smb_mb() inside
> >> > waitqueue_active()??
> >> <snip>
> >> 
> >> There are around 200 occurrences of waitqueue_active() in the kernel
> >> source, and most of the places which use it before wake_up are either
> >> protected by some spin lock, or already has a memory barrier or some
> >> kind of atomic operation before it.
> >> 
> >> Simply adding smp_mb() to waitqueue_active() would incur extra cost in
> >> many cases and won't be a good idea.
> >> 
> >> Another way to solve this problem is to remove the waitqueue_active(),
> >> making the code look like this;
> >> 	if (wq)
> >> 		wake_up_interruptible(wq);
> >> This also fixes the problem because the spinlock in the wake_up*() acts
> >> as a memory barrier and prevents the code from being reordered by the
> >> CPU (and it also makes the resulting code is much simpler).
> > 
> > I might not care which we did, except I don't have the means to test
> > this quickly, and I guess this is some of our most frequently called
> > code.
> > 
> > I suppose your patch is the most conservative approach, as the
> > alternative is a spinlock/unlock in wake_up_interruptible, which I
> > assume is necessarily more expensive than an smp_mb().
> > 
> > As far as I can tell it's been this way since forever.  (Well, since a
> > 2002 patch "NFSD: TCP: rationalise locking in RPC server routines" which
> > removed some spinlocks from the data_ready routines.)
> > 
> > I don't understand what the actual race is yet (which code exactly is
> > missing the wakeup in this case?  nfsd threads seem to instead get
> > woken up by the wake_up_process() in svc_xprt_do_enqueue().)
> 
> Thank you for the reply.  I tried looking into this.
> 
> The callbacks in net/sunrpc/svcsock.c are set up in svc_tcp_init() and
> svc_udp_init(), which are both called from svc_setup_socket().
> svc_setup_socket() is called (indirectly) from lockd, nfsd, and nfsv4
> callback port related code.
> 
> Maybe I'm wrong, but there might not be any kernel code that is using
> the socket's wait queue in this case.

As Trond points out there are probably waiters internal to the
networking code.

--b.