[erlang-questions] Is ei_xreceive_msg() thread safe?

Robert Raschke rtrlists@REDACTED
Tue Aug 27 14:35:57 CEST 2013

Just noticed that my phone email decided not to reply-all.

On Tue, Aug 27, 2013 at 1:32 PM, Robert Raschke <rtrlists@REDACTED>wrote:

> Hi Peter,
> that sounds OK, I guess. It's been ages since I spent any time in the deep
> down details of how the receive/send works, so It's difficult for me to say
> for certain that a mutex around each explicit send is enough. Your theory
> of the receive doing a send internally without the protection of a mutex
> sounds reasonable.
> I don't think you can handle the TICK messages yourself, but you might dig
> in to see if that's possible.
> Another way that comes to mind is to wrap both receive and send in the
> same mutex and have receive be "non-blocking" via the timeout version of
> the function.
> HTH,
> Robby
> On Tue, Aug 27, 2013 at 9:50 AM, Peter Membrey <peter@REDACTED> wrote:
>> Hi Robert,
>> I have a thread pool that receives data from another C library and
>> generates messages for sending to Erlang. Each message is atomic and self
>> contained, so it's not a data stream as such, just a bunch of messages.
>> These can be received and handled in any order at the Erlang side so that
>> should be okay I'd have thought. The mutex protects ei_send() with the
>> intention of ensuring that only one thread can send a message at a time. Of
>> course this assumes that when ei_send() returns, the message has actually
>> been sent...
>> I get data corruption on the Erlang node where it seems as though
>> messages have become mixed up or inter-mingled. All the ei_send() that I do
>> are protected by mutex, and the the main thread of the application simply
>> blocks on ei_xreceive() - so in theory it could reply to a heart beat at
>> any time which maybe gets interspersed with sends from the call back
>> threads because it is not protected by the mutex.
>> I can't think of any other way to do this, so I'd greatly appreciate any
>> guidance you can give me...
>> Kind regards,
>> Peter Membrey
>> ------------------------------
>> *From: *"Robert Raschke" <rtrlists@REDACTED>
>> *To: *"Peter Membrey" <peter@REDACTED>
>> *Sent: *Tuesday, 27 August, 2013 4:39:01 PM
>> *Subject: *Re:[erlang-questions] Is ei_xreceive_msg() thread safe?
>> You need to call ei_xreceive_msg() regularly, it only handles the pings
>> while it is running. Is it possible that you are losing the connection to
>> Erlang because of that (see the discussion about net_ticktime in the kernel
>> app documentation) ?
>> But I also note you say you are starting threads to handle work. Is each
>> one of them simply sending data to Erlang? Wouldn't that mean you're
>> sending randomly interspersed pieces of data? I think you may have to
>> investigate handling the comms back to Erlang in more detail. This may
>> depend a lot on your use case. But simply mutexing the sends doesn't sound
>> right to me.
>> Robby
>>  On Aug 27, 2013 8:26 AM, "Peter Membrey" <peter@REDACTED> wrote:
>>> Hi guys,
>>> I've got a fairly basic C Node set up where I have the main thread
>>> running in a loop with ei_xreceive_msg() and a number of "callback" threads
>>> that execute functions and write data using ei_send() to the shared socket
>>> (connecting to the Erlang node).
>>> Originally I had a lot of data corruption (the Erlang node crashing due
>>> to corrupt data) because of incorrect locking on socket writes. I added
>>> mutexes to the ei_send() calls and this problem seemed to go away.
>>> However I've had a couple of occasions where the system has been quiet
>>> and then suddenly become busy where corrupt data has still been sent to the
>>> Erlang node. I'm positive all the places where I do ei_send() are
>>> protected, but that got me wondering about ei_xreceive_msg().
>>> From what I can find, ei_xreceive_msg() automatically handles heartbeats
>>> for you and I guess that means it will send some sort of reply on that
>>> socket. If the heart beat is being sent at the same time as some other
>>> process tries to write to the socket, is it possible that the two could get
>>> interleaved or something? I would honestly have thought it unlikely but I'm
>>> running out of ideas.
>>> Assuming it's possible, how could I add a mutex in this case? The call
>>> itself blocks, so I can't wrap the whole call in the mutex else nothing
>>> else will be able to send data, and there's no way to pass a mutex into the
>>> call itself. So as far as I can tell, there's no way to protect these
>>> writes and prevent them from getting mixed up with other writes on that
>>> socket.
>>> Does anyone have any ideas? I'm quite willing to accept I could be doing
>>> something pretty stupid, but I'm really out of ideas as to what that might
>>> be...
>>> Thanks in advance!
>>> Kind Regards,
>>> Peter Membrey
>>> _______________________________________________
>>> erlang-questions mailing list
>>> erlang-questions@REDACTED
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