Maybe it's atomic on a single core CPU but it's not going to be safe
with multicore CPU's.
Having thought about it some more, I don't think it's enough to just
protect calls to triggerEvent() because 'fTriggersAwaitingHandling' is
still modified by BasicTaskScheduler::SingleStep().
I'm not just speculating! but this problem can take hours to show it's
self so I need to run the test for a long time to be confident that it's
fixed. I have only protected the triggerEvent() method but I guess the
chance of it failing now will be much lower because the task scheduler
thread is probably waiting in a select() call most of the time.
I will have to implement my own task scheduler class to be completely
confident though. I get that it's difficult to add synchronization in a
portable way, but at least the documentation could be updated to
highlight the problem.
Perhaps you could add an interface for a synchronization object and
users could supply their own platform specific implementations.
On 03/16/2015 11:18 AM, Ross Finlayson wrote:
But I can't see how the implementation of triggerEvent() can be
thread safe, e.g, the last line:
fTriggersAwaitingHandling |= eventTriggerId;
must load, modify and store the value at fTriggersAwaitingHandling
It depends on the CPU architecture. In many (if not most?)
architectures, it'll just be a single instruction.
I'd put a mutex around that instruction, if there were a good portable
way of doing so (i.e., portable across Unix and Windows, and across
both old and new compiler versions).
Feel free to put a mutex around your call to "triggerEvent()" - to see
if that solves your problem. (In fact, you should have done that
first, before speculating on this mailing list :-)
Ross Finlayson
Live Networks, Inc.
http://www.live555.com/
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