Assigning memory barriers in the linux kernel

Question

Robert Paul says that “set_task_state (task, state) sets the given task to the given state. If applicable, it also provides a memory barrier for forced sequencing on other processors (this is only necessary for SMP systems). Otherwise, it is equivalent to task- > state = state

My question is: How can a memory barrier force me to streamline work on other processors?

What does the love of robert mean? Why is this required? What is this order he could talk about? Does he talk about planning queues here?

If so, does each processor in the SMP have a different scheduling queue? I'm confused

Answer 1

CPU, , , , . , . / (, , , , )

1: CPU1: task->state = someModifiedStuff
2: CPU1: changed = 1;
3: CPU2: if (changed)
4: CPU2:  ...

, 1 2. . SMP 1 2, 3 , . , CPU1 2 ( 1), CPU2 3 4, CPU2 , , , CPU1, .

, - 1 2 , .

" ", :

Answer 2

, : , .

- volatile ( ++ ). , (, lock) volatile, - fence ( : . 7.5.5 http://www.intel.com/content/www/us/en/architecture-and-technology/64-ia-32-architectures-optimization-manual.html)

, ?

x = 0;

thread 1:                           thread 2:

a.lock();                           a.lock();
x++;                                x++;
a.unlock();                         a.unlock();

x 2. , . , (a x , , lock() ):

x = 0;

thread 1:                           thread 2:

x++;                                x++;
a.lock();                           a.lock();
a.unlock();                         a.unlock();

x 2 1.