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Re: [PATCH] lib: fix data race in rhashtable_rehash_one

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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.
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[PATCH] lib: fix data race in rhashtable_rehash_one Dmitry Vyukov <dvyukov@google.com> - 2015-09-21 10:10 +0200
  Re: [PATCH] lib: fix data race in rhashtable_rehash_one Eric Dumazet <eric.dumazet@gmail.com> - 2015-09-21 15:40 +0200
    Re: [PATCH] lib: fix data race in rhashtable_rehash_one Eric Dumazet <eric.dumazet@gmail.com> - 2015-09-21 17:00 +0200
      Re: [PATCH] lib: fix data race in rhashtable_rehash_one Eric Dumazet <eric.dumazet@gmail.com> - 2015-09-21 17:20 +0200
      Re: [PATCH] lib: fix data race in rhashtable_rehash_one Dmitry Vyukov <dvyukov@google.com> - 2015-09-21 17:20 +0200
      Re: [PATCH] lib: fix data race in rhashtable_rehash_one Thomas Graf <tgraf@suug.ch> - 2015-09-22 00:30 +0200
        Re: [PATCH] lib: fix data race in rhashtable_rehash_one Eric Dumazet <eric.dumazet@gmail.com> - 2015-09-22 01:10 +0200
          Re: [PATCH] lib: fix data race in rhashtable_rehash_one Thomas Graf <tgraf@suug.ch> - 2015-09-22 10:30 +0200
            Re: [PATCH] lib: fix data race in rhashtable_rehash_one Dmitry Vyukov <dvyukov@google.com> - 2015-09-22 11:00 +0200
          Re: [PATCH] lib: fix data race in rhashtable_rehash_one Herbert Xu <herbert@gondor.apana.org.au> - 2015-09-22 17:20 +0200

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