netdev - Re: [PATCH] inet: frags: Remove unnecessary smp_store_release/READ

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Message-ID: <CANn89iL4xYj5mg3Ydh3c1XwWdrnGQ=uiVvx-xS75oPMaEMJ0Yg@mail.gmail.com>
Date:   Fri, 31 May 2019 10:18:45 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     "Paul E. McKenney" <paulmck@...ux.ibm.com>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Andrea Parri <andrea.parri@...rulasolutions.com>,
        Alan Stern <stern@...land.harvard.edu>,
        David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        syzbot <syzkaller@...glegroups.com>
Subject: Re: [PATCH] inet: frags: Remove unnecessary smp_store_release/READ_ONCE

On Fri, May 31, 2019 at 10:11 AM Paul E. McKenney <paulmck@...ux.ibm.com> wrote:
>
> On Fri, May 31, 2019 at 08:45:47AM -0700, Eric Dumazet wrote:
> >
> >
> > On 5/31/19 7:45 AM, Herbert Xu wrote:
> > > On Fri, May 31, 2019 at 10:24:08AM +0200, Dmitry Vyukov wrote:
> > >>
> > >> OK, let's call it barrier. But we need more than a barrier here then.
> > >
> > > READ_ONCE/WRITE_ONCE is not some magical dust that you sprinkle
> > > around in your code to make it work without locks.  You need to
> > > understand exactly why you need them and why the code would be
> > > buggy if you don't use them.
> > >
> > > In this case the code doesn't need them because an implicit
> > > barrier() (which is *stronger* than READ_ONCE/WRITE_ONCE) already
> > > exists in both places.
> > >
> >
> > More over, adding READ_ONCE() while not really needed prevents some compiler
> > optimizations.
> >
> > ( Not in this particular case, since fqdir->dead is read exactly once, but we could
> > have had a loop )
> >
> > I have already explained that the READ_ONCE() was a leftover of the first version
> > of the patch, that I refined later, adding correct (and slightly more complex) RCU
> > barriers and rules.
> >
> > Dmitry, the self-documentation argument is perfectly good, but Herbert
> > put much nicer ad hoc comments.
>
> I don't see all the code, but let me see if I understand based on the
> pieces that I do see...
>
> o       fqdir_exit() does a store-release to ->dead, then arranges
>         for fqdir_rwork_fn() to be called from workqueue context
>         after a grace period has elapsed.
>
> o       If inet_frag_kill() is invoked only from fqdir_rwork_fn(),
>         and if they are using the same fqdir, then inet_frag_kill()
>         would always see fqdir->dead==true.
>
>         But then it would not be necessary to check it, so this seems
>         unlikely
>

Nope, inet_frag_kill() can be called from timer handler, and there is
already an existing barrier (spinlock) before we call it (also under
rcu_read_lock())

ip_expire(struct timer_list *t)

rcu_read_lock();
spin_lock(&qp->q.lock);
 ... ipq_kill(qp);   -> inet_frag_kill()





> o       If fqdir_exit() does store-releases to a number of ->dead
>         fields under rcu_read_lock(), and if the next fqdir_exit()
>         won't happen until after all the callbacks complete
>         (combination of flushing workqueues and rcu_barrier(), for
>         example), then ->dead would be stable when inet_frag_kill()
>         is invoked, and might be true or not.  (This again requires
>         inet_frag_kill() be only invoked from fqdir_rwork_fn().)
>
> So I can imagine cases where this would in fact work.  But did I get
> it right or is something else happening?
>
>                                                         Thanx, Paul
>