4 ms·

Two operations can be concurrent, even if they are run on the same thread, as long as they are unsequenced, in other words, there is nothing making one happen b

Two operations can be concurrent, even if they are run on the same thread, as long as they are unsequenced, in other words, there is nothing making one happen before the other. That's what the C++ standard says, linked from the wikipedia article you linked. I realize that C++ may not be completely applicable to Python, but this fits my prior understanding of the topic. Since there's no formal semantics to consult here, no actual alternate definition of concurrency, and the definition of data race fits, I can't say that the article is incorrect. This is going long and I need to get dinner, but thank you for the great discussion!

Kranar 5y ago
You have misread what C++/Wikipedia says and are hence incorrect. The Wikipedia article says that actions are potentially concurrent if (they run from two different threads) OR (they are unsequenced AND one operation is performed by a signal handler). I have added the parenthesis to group together the logical clauses. Your omission of the requirement that the operation be performed from within a signal handler is significant and Wikipedia states the following about it: "The parts of this definition relating to signal handlers are idiosyncratic to C++ and are not typical of definitions of data race." Now we can definitely go into the nature of signal handlers in C++, but suffice it to say signal handlers are not considered to be executed from within the same thread as the main program. A signal handler is executed from within a special execution context that is independent of the rest of the program and whose behavior is distinct from typical threads. Suffice to say, other than specific idiosyncrasies, a signal handler is treated as if it were a separate thread of execution (even though technically an implementation does not have to actually spawn a separate thread for a signal handler).

Kranar 5y ago

You have misread what C++/Wikipedia says and are hence incorrect. The Wikipedia article says that actions are potentially concurrent if (they run from two different threads) OR (they are unsequenced AND one operation is performed by a signal handler). I have added the parenthesis to group together the logical clauses. Your omission of the requirement that the operation be performed from within a signal handler is significant and Wikipedia states the following about it: "The parts of this definition relating to signal handlers are idiosyncratic to C++ and are not typical of definitions of data race." Now we can definitely go into the nature of signal handlers in C++, but suffice it to say signal handlers are not considered to be executed from within the same thread as the main program. A signal handler is executed from within a special execution context that is independent of the rest of the program and whose behavior is distinct from typical threads. Suffice to say, other than specific idiosyncrasies, a signal handler is treated as if it were a separate thread of execution (even though technically an implementation does not have to actually spawn a separate thread for a signal handler).