[erlang-questions] dialyzer: possible race condition with insert after lookup

Joel Reymont <>
Sun Oct 23 21:29:38 CEST 2011


I meant to ask how to avoid -this- race condition since Dialyzer also complains about it!

On Oct 23, 2011, at 8:28 PM, Ulf Wiger wrote:

> 
> No, you'll want to stick to the update_counter semantics: the point of using ets:insert_new() and then update_counter() again, was that even if another process does the same thing in parallel, only one will get to insert the zero value; then both will use update_counter(), which will be atomic and thread-safe.
> 
> BR,
> Ulf
> 
> On 23 Oct 2011, at 21:19, Joel Reymont wrote:
> 
>> What about this?
>> 
>>           _ = case ets:lookup(stats, Key1) of
>>                   [{Key1, TS, latency, OldDelta}] ->
>>                       %% have timings, average
>>                       ets:insert(stats, [{Key1, TS, latency, (OldDelta + Delta) / 2}]);
>>                   _ ->
>>                       %% first timing, store
>>                       ets:insert(stats, [{Key1, now(), latency, Delta}])
>>               end;
>> 
>> On Oct 23, 2011, at 8:00 PM, Ulf Wiger wrote:
>> 
>>> 
>>> This might please Dialyzer (I haven't tried):
>>> 
>>> bump(Key, Value) ->
>>> try ets:update_counter(stats, Key, {4, Value})
>>> catch
>>>    error:_ ->
>>>       ets:insert_new(stats, {Key, now(), count, 0}),
>>>       ets:update_counter(stats, Key, {4, Value})
>>> end.
>> 
>> --------------------------------------------------------------------------
>> - for hire: mac osx device driver ninja, kernel extensions and usb drivers
>> ---------------------+------------+---------------------------------------
>> http://wagerlabs.com | @wagerlabs | http://www.linkedin.com/in/joelreymont
>> ---------------------+------------+---------------------------------------
>> 
> 
> Ulf Wiger, CTO, Erlang Solutions, Ltd.
> http://erlang-solutions.com
> 
> 
> 

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---------------------+------------+---------------------------------------
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