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SubjectRe: freezer: should barriers be smp ?
On Wed 2011-04-13 17:02:45, Mike Frysinger wrote:
> On Wed, Apr 13, 2011 at 16:58, Rafael J. Wysocki wrote:
> > On Wednesday, April 13, 2011, Mike Frysinger wrote:
> >> when we suspend/resume Blackfin SMP systems, we notice that the
> >> freezer code runs on multiple cores.  this is of course what you want
> >> -- freeze processes in parallel.  however, the code only uses non-smp
> >> based barriers which causes us problems ... our cores need software
> >> support to keep caches in sync, so our smp barriers do just that.  but
> >> the non-smp barriers do not, and so the frozen/thawed processes
> >> randomly get stuck in the wrong task state.
> >>
> >> thinking about it, shouldnt the freezer code be using smp barriers ?
> >
> > Yes, it should, but rmb() and wmb() are supposed to be SMP barriers.
> >
> > Or do you mean something different?
>
> then what's the diff between smp_rmb() and rmb() ?
>
> this is what i'm proposing:
> --- a/kernel/freezer.c
> +++ b/kernel/freezer.c
> @@ -17,7 +17,7 @@ static inline void frozen_process(void)
> {
> if (!unlikely(current->flags & PF_NOFREEZE)) {
> current->flags |= PF_FROZEN;
> - wmb();
> + smp_wmb();
> }
> clear_freeze_flag(current);
> }
> @@ -93,7 +93,7 @@ bool freeze_task(struct task_struct *p, bool sig_only)
> * the task as frozen and next clears its TIF_FREEZE.
> */
> if (!freezing(p)) {
> - rmb();
> + smp_rmb();
> if (frozen(p))
> return false;

smp_rmb() is NOP on uniprocessor.

I believe the code is correct as is.
Pavel

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