netdev - Re: [PATCH v2 net] ipv4: update fib_info

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Date:   Mon, 17 Jan 2022 01:48:20 -0800
From:   Eric Dumazet <edumazet@...gle.com>
To:     David Laight <David.Laight@...lab.com>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        netdev <netdev@...r.kernel.org>,
        syzbot <syzkaller@...glegroups.com>,
        Ido Schimmel <idosch@...lanox.com>,
        Jiri Pirko <jiri@...lanox.com>
Subject: Re: [PATCH v2 net] ipv4: update fib_info_cnt under spinlock protection

On Sun, Jan 16, 2022 at 7:23 PM David Laight <David.Laight@...lab.com> wrote:
>
> From: Eric Dumazet
> > Sent: 16 January 2022 09:02
> >
> > In the past, free_fib_info() was supposed to be called
> > under RTNL protection.
> >
> > This eventually was no longer the case.
> >
> > Instead of enforcing RTNL it seems we simply can
> > move fib_info_cnt changes to occur when fib_info_lock
> > is held.
> >
> > v2: David Laight suggested to update fib_info_cnt
> > only when an entry is added/deleted to/from the hash table,
> > as fib_info_cnt is used to make sure hash table size
> > is optimal.
>
> Already applied, but
> acked-by: David Laight
>
> ...
> If you are going to add READ_ONCE() markers then one on
> 'fib_info_hash_size' would be much more appropriate since
> this value is used twice.
>
> >       err = -ENOBUFS;
> > -     if (fib_info_cnt >= fib_info_hash_size) {
> > +
> > +     /* Paired with WRITE_ONCE() in fib_release_info() */
> > +     if (READ_ONCE(fib_info_cnt) >= fib_info_hash_size) {
> >               unsigned int new_size = fib_info_hash_size << 1;
> >               struct hlist_head *new_info_hash;
> >               struct hlist_head *new_laddrhash;
> > @@ -1462,7 +1467,6 @@ struct fib_info *fib_create_info(struct fib_config *cfg,
>
> If is also possible for two (or many) threads to decide to
> increase the hash table size at the same time.
>
> The code that moves the items to the new hash tables should
> probably discard the new tables is they aren't larger than
> the existing ones.
> The copy does look safe - just a waste of time.
>
> It is also technically possible (but very unlikely) that the table
> will get shrunk!
> It will grow again on the next allocate.
>
> But this is a different bug.
>

There is no bug.

fib_create_info() is called with RTNL held.



> I also though Linus said that the WRITE_ONCE() weren't needed
> here because the kernel basically assumes the compiler isn't
> stupid enough to do 'write tearing' on word sized items
> (or just write zero before every write).
>

That is not true. READ_ONCE()/WRITE_ONCE() have their own purpose,
we can not assume a compiler will follow arbitrary rules about word sizes.