netdev - Re: [PATCH] lib: fix data race in rhashtable_rehash

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Date:	Mon, 21 Sep 2015 17:10:06 +0200
From:	Dmitry Vyukov <dvyukov@...gle.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	tgraf@...g.ch, netdev@...r.kernel.org,
	LKML <linux-kernel@...r.kernel.org>,
	Kostya Serebryany <kcc@...gle.com>,
	Andrey Konovalov <andreyknvl@...gle.com>,
	Alexander Potapenko <glider@...gle.com>,
	ktsan@...glegroups.com, Paul McKenney <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH] lib: fix data race in rhashtable_rehash_one

On Mon, Sep 21, 2015 at 4:51 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Mon, 2015-09-21 at 06:31 -0700, Eric Dumazet wrote:
>> On Mon, 2015-09-21 at 10:08 +0200, Dmitry Vyukov wrote:
>> > rhashtable_rehash_one() uses plain writes to update entry->next,
>> > while it is being concurrently accessed by readers.
>> > Unfortunately, the compiler is within its rights to (for example) use
>> > byte-at-a-time writes to update the pointer, which would fatally confuse
>> > concurrent readers.
>> >
>> This is bogus.
>>
>> 1) Linux is certainly not working if some arch or compiler is not doing
>> single word writes. WRITE_ONCE() would not help at all to enforce this.
>>
>> 2) If  new node is not yet visible, we don't care if we write
>> entry->next using any kind of operation.
>>
>> So the WRITE_ONCE() is not needed at all.
>>
>>
>>
>> > +   WRITE_ONCE(entry->next, head);
>>
>>
>> The rcu_assign_pointer() immediately following is enough in this case.
>>
>> We have hundred of similar cases in the kernel.
>>
>>
>
> The changelog and comment are totally confusing.
>
> Please remove the bogus parts in them, and/or rephrase.
>
> 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.

I have not looked in detail yet, but the NULLS recomputation uses
new_hash, which obviously wasn't available when the value was
previously computed. Don't know yet whether it is important or not.



>
> diff --git a/lib/rhashtable.c b/lib/rhashtable.c
> index cc0c69710dcf..0a29f07ba45a 100644
> --- a/lib/rhashtable.c
> +++ b/lib/rhashtable.c
> @@ -187,10 +187,7 @@ static int rhashtable_rehash_one(struct rhashtable *ht, unsigned int old_hash)
>         head = rht_dereference_bucket(new_tbl->buckets[new_hash],
>                                       new_tbl, new_hash);
>
> -       if (rht_is_a_nulls(head))
> -               INIT_RHT_NULLS_HEAD(entry->next, ht, new_hash);
> -       else
> -               RCU_INIT_POINTER(entry->next, head);
> +       RCU_INIT_POINTER(entry->next, head);
>
>         rcu_assign_pointer(new_tbl->buckets[new_hash], entry);
>         spin_unlock(new_bucket_lock);
>
>
> --
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-- 
Dmitry Vyukov, Software Engineer, dvyukov@...gle.com
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