Is there a reason a thread shouldn't access other thread's stack?

Question

I was just playing with Intel Parallel inspector on my project, and it displays a warning:

One or more threads in the application accessed the stack of another thread. This may indicate one or more bugs in your application.

I do indeed have some objects that are allocated on stack shared between threads. I don't see why this is a problem. Any hints?

Answer 1

It's not wrong, it's just possibly wrong. Tools like Intel Parallel Inspector that provide additional diagnostics for your program must make a tradeoff between false positives and false negatives, in this case, it seems that the developers thought that accessing the stack of another thread was much more likely to be an error (low false positive rate if reported) than not (high false negative rate if not reported).

Valgrind is another example of a tool that can signal errors in code that is correct.

The real question here is, "what is the other thread doing?" If you think, "maybe it will return from that function and the stack frame will be invalid," then you are doing parallel programming wrong. No answer about multithreaded behavior should be qualified with "maybe". You had better make sure that that thread doesn't return, for example, by making it wait on a semaphore or condition variable, or by making it join with the other threads.

Discussion

Pubby: "AFAIK it's hugely inefficient."

The only reason it would be inefficient is because you might have multiple cores modifying the same cache lines, which is the same problem you have with other kinds of shared memory.

Collin: How do you know the stack frame is still good in another thread?

If you use something in multiple threads, you use some kind of synchronization mechanism to ensure that it's not modified in an invalid way. This situation is no different.

H2CO3: Well, is there a reason you should not walk into another person's house?

If we're going to play with analogies, I'd say that the process is the house, and each of the threads are people in the house. If Dave keeps a list of chores in his room, I'll go into his room every time I need to look at the list. If he stops doing that, he'd better tell me or else I'll start writing on random pieces of paper on his desk.

Conclusion

It's a matter of style whether this program behavior is acceptable or not.