netdev - Re: [PATCH net-next 11/11] inet: frags: rework rhashtable dismantle

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Message-ID: <CANn89i+NfFLHDthLC-=+vWV6fFSqddVqhnAWE_+mHRD9nQsNyw@mail.gmail.com>
Date:   Tue, 28 May 2019 06:31:00 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Herbert Xu <herbert@...dor.apana.org.au>
Cc:     David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Eric Dumazet <eric.dumazet@...il.com>,
        syzbot <syzkaller@...glegroups.com>
Subject: Re: [PATCH net-next 11/11] inet: frags: rework rhashtable dismantle

On Mon, May 27, 2019 at 11:34 PM Herbert Xu <herbert@...dor.apana.org.au> wrote:
>
> Hi Eric:
>
> Eric Dumazet <edumazet@...gle.com> wrote:
> >
> > +void fqdir_exit(struct fqdir *fqdir)
> > +{
> > +       fqdir->high_thresh = 0; /* prevent creation of new frags */
> > +
> > +       /* paired with READ_ONCE() in inet_frag_kill() :
> > +        * We want to prevent rhashtable_remove_fast() calls
> > +        */
> > +       smp_store_release(&fqdir->dead, true);
> > +
> > +       INIT_RCU_WORK(&fqdir->destroy_rwork, fqdir_rwork_fn);
> > +       queue_rcu_work(system_wq, &fqdir->destroy_rwork);
> > +
> > +}
>
> What is the smp_store_release supposed to protect here? If it's
> meant to separate the setting of dead and the subsequent destruction
> work then it doesn't work because the barrier only protects the code
> preceding it, not after.
>

This smp_store_release() is a left over of the first version of the patch, where
there was no rcu grace period enforcement.

I do not believe there is harm letting this, but if you disagree
please send a patch ;)

Thanks