Re: [linux-pm] [uclinux-dist-devel] freezer: should barriers be smp?

[Alan Stern]

On Wed, 13 Apr 2011, Mike Frysinger wrote:

> On Wed, Apr 13, 2011 at 17:05, Pavel Machek wrote:
> > On Wed 2011-04-13 17:02:45, Mike Frysinger wrote:
> 
> >> 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)
> >> Â{
> >> Â Â if (!unlikely(current->flags & PF_NOFREEZE)) {
> >> Â Â Â Â current->flags |= PF_FROZEN;
> >> - Â Â Â wmb();
> >> + Â Â Â smp_wmb();
> >> Â Â }
> >> Â Â clear_freeze_flag(current);
> >> Â}
> >> @@ -93,7 +93,7 @@ bool freeze_task(struct task_struct *p, bool sig_only)
> >> Â Â Â* the task as frozen and next clears its TIF_FREEZE.
> >> Â Â Â*/
> >> Â Â if (!freezing(p)) {
> >> - Â Â Â rmb();
> >> + Â Â Â smp_rmb();
> >> Â Â Â Â if (frozen(p))
> >> Â Â Â Â Â Â return false;
> >
> > smp_rmb() is NOP on uniprocessor.
> >
> > I believe the code is correct as is.
> 
> that isnt what the code / documentation says.  unless i'm reading them
> wrong, both seem to indicate that the proposed patch is what we
> actually want.

The existing code is correct but it isn't optimal.

wmb() and rmb() are heavy-duty operations, and you don't want to call
them when they aren't needed.  That's exactly what smp_wmb() and
smp_rmb() are for -- they call wmb() and rmb(), but only in SMP 
kernels.

Unless you need to synchronize with another processor (not necessarily 
a CPU, it could be something embedded within a device), you should 
always use smp_wmb() and smp_rmb() rather than wmb() and rmb().

Alan Stern

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