Re: [erlang-questions] Port Driver. erl_drv_thread_join and ErlDrvTid reuse.

[Rickard Green]

>>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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[Zhemzhitsky Sergey]

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

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[Rickard Green]

The tid cannot be prematurely reused in this case. The EDEADLK is due to
some other reason.

Are you calling driver_failure_atom() from your thread? That would
explain this issue.

Regards,
Rickard

[Zhemzhitsky Sergey]

Hi Rickard,

>> Are you calling driver_failure_atom() from your thread? That would explain this issue.

Exactly. Could you explain a little bit more why it can cause this issue (I suppose it's due to it is not thread safe)?


Best Regards,
Sergey Zhemzhitsky
Phone. +7 495 2580500 ext. 1246

[Rickard Green]

On 10/11/2012 03:11 PM, Zhemzhitsky Sergey wrote:
> Hi Rickard,
>
>>> Are you calling driver_failure_atom() from your thread? That would explain this issue.
>
> Exactly. Could you explain a little bit more why it can cause this issue (I suppose it's due to it is not thread safe)?
>

Yes, it is because driver_failure_atom() is not thread safe. Apart from
misc thread unsafe modifications of the port internal state,
driver_failure_atom() can call the stop() callback which means that your
thread will end up trying to join with itself.

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

[Zhemzhitsky Sergey]

Thanks a lot. Now it's clear enough.

Best Regards,
Sergey

-----Original Message-----
From: Rickard Green [mailto:ric...@erlang.org]
Sent: Thursday, October 11, 2012 6:13 PM
To: Zhemzhitsky Sergey
Cc: Erlang Questions
Subject: Re: [erlang-questions] Port Driver. erl_drv_thread_join and ErlDrvTid reuse.

On 10/11/2012 03:11 PM, Zhemzhitsky Sergey wrote:
> Hi Rickard,
>
>>> Are you calling driver_failure_atom() from your thread? That would explain this issue.
>
> Exactly. Could you explain a little bit more why it can cause this issue (I suppose it's due to it is not thread safe)?
>

Yes, it is because driver_failure_atom() is not thread safe. Apart from misc thread unsafe modifications of the port internal state,
driver_failure_atom() can call the stop() callback which means that your thread will end up trying to join with itself.

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

_______________________________________________________
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