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

From: Linus Torvalds
Date: Fri Apr 02 2010 - 15:47:35 EST

On Fri, 2 Apr 2010, Jason Wessel wrote:
> 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();

(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.

The fact that maybe some ARM6 cache coherency implementation is pure and
utter sh*t and never sees the changes without the same instruction that
happens to be a memory barrier on that architecture does not make that
cpu_relax() any more about memory barriers.

Similarly, the fact that P4's wanted cpu_relax() in order to not overheat
and cause slowdowns has _nothing_ to do with anything.

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.

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