[PATCH 00/12] libcamera: Hardening against thread race conditions

[Milan Zamazal]

Laurent Pinchart <laurent.pinchart at ideasonboard.com> writes:

> Even if V4L2VideoDevice isn't addressed directly by this series, and the
> bug report is related to usage of the V4L2VideoDevice class, I believe
> the issue in the soft ISP code will be caught by the assertions added to
> the EventNotifier class. Milan, would you be able to test this, and
> confirm that libcamera now complains loudly ?

Nice cleanup, thank you!  Yes, this complains immediately and loudly in the
given case.

Now a related question is how to deal with threads and instances of classes that
don't inherit Object.

I think V4L2VideoDevice::queueBuffer is a good example.  V4L2VideoDevice is
thread-bound but doesn't inherit Object.  So the caller, unless it has created
the given instance itself (and being thread-bound itself or tracking its initial
thread), has no idea about the thread a V4L2VideoDevice instance is bound too,
even less how to call it there, and even less without worrying about
implications of asynchronous calls.

This means ensuring most things run in the same thread.  This may be easy in
some cases and less fine in other ones.  It discourages using multiple threads,
not sure whether this is a good thing or not.  It's not robust in any case.

A possible alternative would be making less things thread-bound.  I suppose
there are reasons why this is not happening in libcamera.

Another alternative would be to use Object (as a direct base class or in
wrappers) or Thread (together with initial thread tracking) to call the right
thread in more places.  It would be limited to situations where possibly
asynchronous invocations don't harm if nothing else.

The call chain in the software ISP branch, DebayerCpu -> SwIspSimple ->
SimpleCameraData -> V4L2VideoDevice (thread-bound) -> EventNotifier (inheriting
Object), must be changed to eventually invoke V4L2VideoDevice::queueBuffer in
the right thread.  Is the recommended practice to examine thoroughly what's
running where or are there any simpler options?

Thanks,
Milan