From: Thomas Graf <tgraf@suug.ch>
To: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Dmitry Vyukov <dvyukov@google.com>,
netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
kcc@google.com, andreyknvl@google.com, glider@google.com,
ktsan@googlegroups.com, paulmck@linux.vnet.ibm.com
Subject: Re: [PATCH] lib: fix data race in rhashtable_rehash_one
Date: Tue, 22 Sep 2015 00:25:38 +0200 [thread overview]
Message-ID: <20150921222538.GA31246@pox.localdomain> (raw)
In-Reply-To: <1442847108.29850.56.camel@edumazet-glaptop2.roam.corp.google.com>
On 09/21/15 at 07:51am, Eric Dumazet wrote:
> The important part here is that we rehash an item, so we need to make
> sure to maintain consistent ->next field, and need to prevent compiler
> from using ->next as a temporary variable.
>
> ptr->next = 1UL | ((base + offset) << 1);
>
> Is dangerous because compiler could issue :
>
> ptr->next = (base + offset);
>
> ptr->next <<= 1;
>
> ptr->next += 1UL;
>
> Frankly, all this looks like an oversight in this code.
>
> Not sure why the NULLS value is even recomputed.
The hash of the chain is part of the NULLS value. Since the
entry might have been moved to a different chain, the NULLS
value must be recalculated to contain the proper hash.
However, nobody is using the hash today as far as I can
see so we could as well just remove it and use the base
value only for the nulls marker.
next prev parent reply other threads:[~2015-09-21 22:25 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-09-21 8:08 [PATCH] lib: fix data race in rhashtable_rehash_one Dmitry Vyukov
2015-09-21 13:31 ` Eric Dumazet
2015-09-21 14:51 ` Eric Dumazet
2015-09-21 15:10 ` Dmitry Vyukov
2015-09-21 15:15 ` Eric Dumazet
2015-09-21 22:25 ` Thomas Graf [this message]
2015-09-21 23:03 ` Eric Dumazet
2015-09-22 8:19 ` Thomas Graf
2015-09-22 8:52 ` Dmitry Vyukov
2015-09-22 15:18 ` Herbert Xu
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