[erlang-questions] What API should be used in port-drivers to guarantee thread-safety of driver_send_term and driver_output_term?

[Rickard Green]

As Lukas said, this is not something you can solve in your driver. It would require changes in the emulator.

driver_send_term() can of course be made thread safe also in the non-smp emulator. Initially we also planned on doing so, but after looking closer at it we dropped that idea. This since it would pull in too much stuff from the smp emulator. One way to do it could be to replace "erl -smp disable" with "erl -smp enable +S1" :-)

The way to send information from threads to Erlang processes in the non-smp case is something like the following using a thread safe queue and a pipe. Create a pipe and call driver_select(port, fd, ERL_DRV_READ, 1) to show your interest in the read end of the pipe. When a thread has enqueued an element on the queue it should also write a byte on the write end of the pipe (note that it is important that this is done *after* the enqueue operation). This will trigger a call to the ready_input(drv_data, fd) callback by the scheduler thread. In the ready_input() callback you read away the byte written on the pipe and then dequeue the element, create a message and send it to the process using driver_[send|output]_term(). In the windows case you need to use an event instead of a pipe.

Also, note that even though driver_send_term() is thread safe in the smp case, driver_output_term() is *not*.

Regards,
Rickard Green, Erlang/OTP, Ericsson AB

On Sep 5, 2012, at 6:37 PM, Lukas Larsson wrote:

> Hello,
> 
> I do not think[1] this is a problem which you can solve in your
> driver. It is probably the internals of the Erlang VM which are not
> threadsafe, i.e. if there are no locks around message queues to
> processes in the non-smp emulator. So while sending data to a process,
> that same process could be reading data from the queue, and voila you
> get a segfault. So in order to fix this I think you would have to
> patch the emulator it self.
> 
> Lukas
> 
> [1]: Note that I haven't actually checked the relevant code, I have
> divined this from what I know of the emulator.
> 
> On Wed, Sep 5, 2012 at 5:18 PM, Zhemzhitsky Sergey
> <Sergey_Zhemzhitsky@REDACTED> wrote:
>> Hi erlang gurus,
>> 
>> 
>> 
>> We’re developing erlang port driver that will send different terms to some
>> processes from the separate threads using driver_send_term.
>> 
>> Each native thread started in the port driver (by means of
>> erl_drv_thread_create) will be associated with only one erlang process and
>> will send messages only to it.
>> 
>> 
>> 
>> As a rule the documentation for driver_send_term says that it’s not
>> thread-safe when smp is disabled, so we have tested the driver with disabled
>> smp support and found that from time to time it leads to segfaults.
>> 
>> 
>> 
>> So, the question is what is the proper way to make calls to driver_send_term
>> (and driver_output_term) thread-safe?
>> 
>> Will driver_pdl_lock, driver_pdl_unlock help?
>> 
>> Will wrapping all the calls to driver_send_term (and driver_output_term)
>> with erl_drv_mutex_lock and erl_drv_mutex_unlock help?
>> 
>> 
>> 
>> 
>> 
>> Best Regards,
>> 
>> Sergey
>> 
>> 
>> 
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