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

[Trond Myklebust]

On Fri, Oct 9, 2015 at 5:18 PM, J. Bruce Fields <bfields@xxxxxxxxxxxx> wrote:
>
> On Fri, Oct 09, 2015 at 06:29:44AM +0000, Kosuke Tatsukawa wrote:
> > Neil Brown wrote:
> > > Kosuke Tatsukawa <tatsu@xxxxxxxxxxxxx> 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().)
>

Those threads still use blocking calls for sendpage() and sendmsg(),
so presumably they may be affected.
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