[erlang-questions] : driver_entry stop() and driver_async() interaction

Dave Smith dizzyd@REDACTED
Thu Dec 18 15:36:34 CET 2008

>>>> Is it the responsibility of the code in the stop() callback to call
>>>> driver_async_cancel() on each outstanding async work item, or will this
>>>> be done automatically by the emulator before the call to stop()?

Yes, it is up to the driver to cancel outstanding items. In wrestling
with this recently, I found that the best approach was to cancel all
the items and then queue up a sentinel callback that triggers a
condition variable. I then wait for that cv to get signaled and know
at that point that the driver is clear to shutdown. This approach
seemed to work quite well on a 8 proc box with reasonable load --
YMMV. :)

>>>> If this is the responsibility of the code in stop(), is it guaranteed
>>>> that no async work item will be executing or scheduled during the call
>>>> to the stop() callback?
>>>> If no guarantee is made, is holding the PDL necessary and sufficient to
>>>> guarantee this?

I researched this, and I believe that the answer is "no". Scheduling
the async work item does increment the PDL ref count when the item is
_inserted_ into the queue. Once the item is in the queue, all bets are
off -- the only guarantee you have is that the PDL won't disappear
(i.e. you can safely lock/unlock it). The PDL says nothing about
whether or not the async work item is executing during stop(). It
seems to be the responsibility of the driver author to sort out these
intricate timing issues.

erts/emulator/beam/erl_async.c is where all the code of interest resides.

It's entirely possible that I have made gross errors in my reading of
the emulator code and would happily receive instruction from one of
the core VM team. :)


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