> On Aug 11, 2015, at 10:42 AM, Jarno Rajahalme <[email protected]> wrote:
>
>
>> On Aug 10, 2015, at 6:46 PM, Ethan J. Jackson <[email protected]> wrote:
>>
>> From: Ethan Jackson <[email protected]>
>>
>> Since revalidator_sweep() doesn't hold the ukey mutex for each full
>> loop iteration, it's theoretically possible that two threads may
>> call ukey_delete() on the same ukey. If this happens, they both will
>> attempt to remove the ukey from he cmap, causing the loser of the race
>> to fail.
>>
>> Signed-off-by: Ethan J. Jackson <[email protected]>
>> ---
>> ofproto/ofproto-dpif-upcall.c | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/ofproto/ofproto-dpif-upcall.c b/ofproto/ofproto-dpif-upcall.c
>> index 0f2e186..fddb535 100644
>> --- a/ofproto/ofproto-dpif-upcall.c
>> +++ b/ofproto/ofproto-dpif-upcall.c
>> @@ -2076,7 +2076,6 @@ revalidator_sweep__(struct revalidator *revalidator,
>> bool purge)
>> flow_exists = ukey->flow_exists;
>> seq_mismatch = (ukey->dump_seq != dump_seq
>> && ukey->reval_seq != reval_seq);
>> - ovs_mutex_unlock(&ukey->mutex);
>>
>> if (flow_exists
>> && (purge
>
> There is a call to push_ukey_ops() between these blocks. It calls
> push_ukey_ops__(), which will take a lock on the ukeys, including the last
> one added and still locked, so this would result in double locking.
>
I missed the fact that handle_missed_revalidation() in here also takes the ukey
lock.
Jarno
> This could be resolved by moving the “ if (n_ops == REVALIDATE_MAX_BATCH)”
> block after the (moved) ukey unlock, similarly to the remainder ops handling
> at the end. However, I’m not sure if this would defeat the purpose of this
> patch.
>
> Also, it seems that generally the umap lock is taken first and ukey locks
> after, so there is a possibility of a deadlock if the locks are taken in the
> reverse order, like in this patch.
>
> It would be really great if we could employ clang thread safety analysis more
> than we do in this file currently, but I’m not sure if that is possible.
>
> Jarno
>
>> @@ -2095,6 +2094,7 @@ revalidator_sweep__(struct revalidator *revalidator,
>> bool purge)
>> ukey_delete(umap, ukey);
>> ovs_mutex_unlock(&umap->mutex);
>> }
>> + ovs_mutex_unlock(&ukey->mutex);
>> }
>>
>> if (n_ops) {
>> --
>> 2.1.0
>>
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>
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