try this code block in one thread
int a = 0;
a = 1; //the thread is running on cpu A
a? //the thread is switched and running on cpu B
so a can be 0 ?
1 Answer
No, a must be 1 because there is a happens-before relationship between the assignment a = 1 and the value of a being read later by the same thread.
If x and y are actions of the same thread and x comes before y in program order, then hb(x, y).
8 Comments
Joni
When a context switch happens, either the value of
a is written back to heap, or its value is stored in the process control block. See also stackoverflow.com/questions/30774550/…
bEn
so the system needs to track all the variables that the thread modified?
bEn
if there are two thread switched, they both write 'a' back to heap? it seems the happens-before does not work
Joni
Yes but typically it doesn't track individual variables because a variable is a programmer concept. Instead the memory management unit tracks cache lines. On the other hand, the variable may be stored in a register, and not be present on the heap at all.
Joni
In your code,
a is a local variable. That means each thread has a copy of its own of a. |
ashared? How is it updated and by what threads? Without proper notation this is highly bad question