Experimental Physics and
Industrial Control System

Dear all,

In the Node.js single-threaded environment, if I load libca.so and call ca_clear_channel() after the user callback execution of ca_array_put_callback(), sometimes a deadlock occurs. After printing logs, I found that the code blocked at Line 391 of https://github.com/epics-base/epics-base/blob/7.0/modules/ca/src/client/access.cpp

390        //
391        CallbackGuard cbGuard ( cac.cbMutex );
392        epicsGuard < epicsMutex > guard ( cac.mutex );
393        pChan->destructor ( *cac.pCallbackGuard.get(), guard );
394        cac.oldChannelNotifyFreeList.release ( pChan );

In Channel Access reference manual, the description of ca_clear_channel() includes the following info,

Note: This operation blocks until any user callbacks for this channel have run to completion. If callbacks take a lock (mutex) then it is the user's responsibility to ensure that this lock is not held when ca_clear_channel() is called, otherwise a deadlock may ensue.

So my question is how to prevent the deadlock from users' side. In the following training link, the approach is to wait in ca_pend_event() for the callbacks to occur. Is there any other approach or idea besides just waiting enough time for callback to finish?

https://epics.anl.gov/docs/GSWE/developtools/EPICSTraining.IntroductionToChannelAccessClients.Handouts.pdf

Thanks,

Lin

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