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SubjectRe: [PATCH 4/5] kgdb: Use atomic operators which use barriers
Hi!

> > Russell had this thread:
> > http://permalink.gmane.org/gmane.linux.ports.arm.kernel/75717
>
> 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.

...so 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?
Pavel
--
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html


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