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Message from discussion Port Driver. erl_drv_thread_join and ErlDrvTid reuse.
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Zhemzhitsky Sergey  
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 More options Oct 11 2012, 8:21 am
From: Zhemzhitsky Sergey <sergey_zhemzhit...@sberbank-cib.ru>
Date: Thu, 11 Oct 2012 12:21:12 +0000
Local: Thurs, Oct 11 2012 8:21 am
Subject: Re: [erlang-questions] Port Driver. erl_drv_thread_join and ErlDrvTid reuse.
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Hello Rickard,

Thanks a lot for clarification.

I'm doing erl_drv_thread_join() only once and only for threads created by erl_drv_thread_create().

The error only happens from time to time in the following scenario:

1. Erlang process, I send messages from the ddriver to, dies for some reason
2. Driver callback process_exit fires.
3. From the driver thread I'm trying to send the final message to the dead erlang process with driver_send_term
4. As the erlang process is dead, driver_send_term fails
5. As driver_send_term fails I'm doing driver_failure_atom to kill linked process.

Here two things happen:

6.1 (*stop)(ErlDrvData drv_data) executes as linked process has been killed by driver_failure_atom
6.2 Thread, started in the driver, exists

When (*stop)(ErlDrvData drv_data) executes erl_drv_thread_join is called for all the threads started in the driver.

So something wrong happens that leads to tid reuse.

Best Regards,
Sergey

-----Original Message-----
From: Rickard Green [mailto:rick...@erlang.org]
Sent: Wednesday, October 10, 2012 11:13 PM
To: Zhemzhitsky Sergey

Cc: Erlang Questions
Subject: Re: [erlang-questions] Port Driver. erl_drv_thread_join and ErlDrvTid reuse.

>>From time to time erl_drv_thread_join returns error EDEADLK=35, i.e. the current thread (scheduler thread) tries to join itself.

> According to the documentation “A Thread identifier may be reused very quickly after a thread has terminated. Therefore, if a thread corresponding to one of the involved thread identifiers has terminated since the thread identifier was saved, the result of erl_drv_equal_tids() might not give the expected result.”

> I suppose that thread terminates earlier then erl_drv_thread_join call happens, so ErlDrvTid is already reused.

This reuse will not cause problems for erl_drv_thread_join() as long as it is used correctly (a tid wont be reused until after the thread has been joined). erl_drv_thread_join() will also refuse to join threads not created by erl_drv_thread_create(), and would in case the scheduler thread tried to join itself fail with EINVAL.

> So the question is how to use erl_drv_thread_join properly and how to guarantee that the saved ErlDrvTid value points to the same data that was returned from erl_drv_thread_create?

It is important that the thread is joined once and *only* once. Are you sure that you don't do two calls to erl_drv_thread_join() for the same thread?

Regards,
Rickard
--
Rickard Green, Erlang/OTP, Ericsson AB.

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