netdev - Re: rhashtable: Fix reader/rehash race

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Message-ID: <20150312133749.GB21620@casper.infradead.org>
Date:	Thu, 12 Mar 2015 13:37:49 +0000
From:	Thomas Graf <tgraf@...g.ch>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	netdev@...r.kernel.org
Subject: Re: rhashtable: Fix reader/rehash race

On 03/12/15 at 10:07pm, Herbert Xu wrote:
> There is a potential race condition between readers and the rehasher.
> In particular, the rehasher could have started a rehash while the
> reader finishes a scan of the old table but fails to see the new
> table pointer.
> 
> This patch closes this window by adding smp_wmb/smp_rmb.
>     
> Signed-off-by: Herbert Xu <herbert@...dor.apana.org.au>

Are you sure this is sufficient? I think it is still possible
for a rehash to preempt a reader in between:

        tbl = rht_dereference_rcu(ht->future_tbl, ht);
        if (unlikely(tbl != old_tbl))

in which case old entries will be moved to the new table
without the reader seeing them. I don't see how ensuring order
solves this.
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