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Subject: Re: [OMPI devel] about btl/scif thread cancellation (#4616 / r31738)
From: Ralph Castain (rhc_at_[hidden])
Date: 2014-05-14 10:55:54


Couple of suggestions:

* detect that this is an older scif lib and just don't build or enable the scif btl

* have a flag that indicates "you should exit", and then tickle the fd so scif_poll exits

Ralph

On May 14, 2014, at 7:45 AM, Nathan Hjelm <hjelmn_at_[hidden]> wrote:

> Looks like this is a scif bug. From the documentation:
>
> scif_poll() waits for one of a set of endpoints to become ready to perform an I/O operation;
> it is syntactically and semantically very similar to poll() . The SCIF functions on which
> scif_poll() waits are scif_accept(), scif_send(), and scif_recv(). Consult the SCIF
> API reference manuals for details on scif_poll() usage.
>
> So, if it is indeed similar to poll() it should wake up when the file
> descriptor is closed.
>
> Since that is not the case I will look through the documentation and see
> if there is a way other than pthread_cancel.
>
> -Nathan
>
> On Wed, May 14, 2014 at 11:18:05AM +0900, Gilles Gouaillardet wrote:
>> Folks,
>>
>> i would like to comment on r31738 :
>>
>>> There is no reason to cancel the listening thread. It should die
>>> automatically when the file descriptor is closed.
>> i could not agree more
>>> It is sufficient to just wait for the thread to exit with pthread join.
>> unfortunatly, at least in my test environment (an outdated MPSS 2.1) it
>> is *not* :-(
>>
>> this is what i described in #4615
>> https://svn.open-mpi.org/trac/ompi/ticket/4615
>> in which i attached scif_hang.c that evidences that (at least in my
>> environment)
>> scif_poll(...) does *not* return after scif_close(...) is closed, and
>> hence the scif pthread never ends.
>>
>> this is likely a bug in MPSS and it might have been fixed in earlier
>> release.
>>
>> Nathan, could you try scif_hang in your environment and report the MPSS
>> version you are running ?
>>
>>
>> bottom line, and once again, in my test environment, pthread_join (...)
>> without pthread_cancel(...)
>> might cause a hang when the btl/scif module is released.
>>
>>
>> assuming the bug is in old MPSS and has been fixed in recent releases,
>> what is the OpenMPI policy ?
>> a) test the MPSS version and call pthread_cancel() or do *not* call
>> pthread_join if buggy MPSS is detected ?
>> b) display an error/warning if a buggy MPSS is detected ?
>> c) do not call pthread_join at all ? /* SIGSEGV might occur with older
>> MPSS, it is in MPI_Finalize() so impact is limited */
>> d) do nothing, let the btl/scif module hang, this is *not* an OpenMPI
>> problem after all ?
>> e) something else ?
>>
>> Gilles
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