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SubjectRe: [RFC][PATCH v2] sched/rt: Use IPI to trigger RT task push migration instead of pulling
On Thu, 26 Feb 2015 08:45:59 +0100
Peter Zijlstra <peterz@infradead.org> wrote:

> On Wed, Feb 25, 2015 at 12:50:15PM -0500, Steven Rostedt wrote:
> > It can't be used for state?
> >
> > If one CPU writes "zero", and the other CPU wants to decide if the
> > system is in the state to do something, isn't a rmb() fine to use?
> >
> >
> > CPU 1:
> >
> > x = 0;
> > /* Tell other CPUs they can now do something */
> > smp_wmb();
> >
> > CPU 2:
> > /* Make sure we see current state of x */
> > smp_rmb();
> > if (x == 0)
> > do_something();
> >
> > The above situation is not acceptable?
>
> Acceptable is just not the word. It plain doesn't work that way.

Thinking about this more, is it because a wmb just forces the CPU to
write everything before this before it writes anything after it. That
is, the writes themselves can happen at a much later time. Does a plain
mb() work the same way if there are no reads required?

>
> > Otherwise, we fail to be able to do_something() when it is perfectly
> > fine to do so.
>
> Can't be helped.

What about using atomic_t?

Note, my latest code doesn't have any of this, but I just want to
understand the semantics of these operations a bit better.

-- Steve


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