Re: [PATCH 4/5] kgdb: Use atomic operators which use barriers

From: Pavel Machek
Date: Mon Apr 05 2010 - 05:21:44 EST


> > Russell had this thread:
> >
> Russell is wrong.
> Yes, originally it was about P4's overheating. But let me repeat: the fact
> is, this _is_ valid kernel code:
> kernel/sched.c- while (task_is_waking(p))
> kernel/sched.c: cpu_relax();

And this is valid (but ugly and not optimal) kernel code:

kernel/sched.c- while (task_is_waking(p))
kernel/sched.c: asm volatile("" :: "memory");

> (where that "task_is_waking()" is simply doing two regular reads, and
> expects another CPU to be changing them).
> This has _nothing_ to do with memory barriers, or with overheating.
> All that matters is that the above kind of while loop must work. The
> architecture needs to do whatever it needs to do to make it work. End of
> discussion. If on ARM6 that means "smp_mb()", then that's an ARM6
> implementation issue. I don't think inserting smp_mb() into cpu_relax() and udelay()
and similar can ever fix the problem fully.

Run smp_mb() from periodic interrupt?
(cesky, pictures)
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