Re: Matlab 2020b crashes with labCA 3.7.2

["Johnson, Andrew N. via Core-talk" <core-talk at aps.anl.gov>]

On Mar 5, 2021, at 8:50 AM, Till Straumann via Tech-talk <tech-talk at aps.anl.gov> wrote:

The deadlock happens when matlab
 - loads a shared object (or library)
 - AND the shared object executes some initialization code (e.g., constructors of global objects defined in the library)
 - AND the initialization code calls 'pthread_join()'. 'pthread_join()' then never returns.

Note that if 'ordinary' code in the shared object (i.e., as opposed to initialization code) uses 'pthread_join()' then
that works fine.

EPICS' libCom does use 'pthread_join()' during initialization and is therefore affected.

At this point I can suggest two possible work-arounds (using one of them is sufficient):

1.) Use an EPICS-base build with posix priority scheduling disabled. This avoids a section of initialization
     code which calls 'pthread_join()'

The init code that Till is referring to has been in osi/os/posix/osdThread.c since 2011:

static void findPriorityRange(commonAttr *a_p)

{

priAvailable arg;

pthread_t    id;

void         *dummy;

int          status;

    arg.policy = a_p->schedPolicy;

    arg.ok = 0;

    status = pthread_create(&id, 0, find_pri_range, &arg);

    checkStatusOnceQuit(status, "pthread_create","epicsThreadInit");

    status = pthread_join(id, &dummy);

    checkStatusOnceQuit(status, "pthread_join","epicsThreadInit");

    a_p->minPriority = arg.min_pri;

    a_p->maxPriority = arg.max_pri;

    a_p->usePolicy = arg.ok;

}

When posix priority scheduling is enabled this gets called from inside the once() routine that the Posix epicsThreadInit() implementation runs using pthread_once(). I don’t see an easy way to avoid doing that initialization here; it might be possible by adding more lazy-init logic to only look up the min and max scheduling priorities when they are actually needed, but I suspect that would still be before the end of the initialization code.

Hence not something we should try to fix IMHO.

- Andrew

-- 

Complexity comes for free, simplicity you have to work for.