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Date: Fri, 23 Jun 2006 10:30:18 +0200
From: Andreas Mohr <andi@rhlx01.fht-esslingen.de>
To: Chuck Ebbert <76306.1226@compuserve.com>
Cc: linux-kernel <linux-kernel@vger.kernel.org>, Andrew Morton <akpm@osdl.org>,
       Dave Jones <davej@redhat.com>
Subject: Re: [patch] i386: cpu_relax() in crash.c and doublefault.c
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Hi,

On Fri, Jun 23, 2006 at 03:40:25AM -0400, Chuck Ebbert wrote:
> Add cpu_relax() to infinite loops in crash.c and
> doublefault.c.  This is the safest change.

Thanks for your continued work on this!

What's the reasoning for not running halt() in doublefault_fn()?


In order to consolidate those places to safely halt a CPU,
I could think of (possibly in a header file):

/* very, very safely halt CPU:
   - do minimal checking in case CPU might already be overheated (unreliable!)
     (also use inlining to avoid call overhead on an unreliable CPU)
   - try to use halt() and cpu_relax() very liberally to keep the crashed
     CPU as cool as possible (crash might have happened due to CPU fan failure!)
     While ACPI specifies CPU shutdown on over-temperature, we really don't
     want to rely on this since it might be broken or we simply don't use ACPI
     mode at all...
*/ 
inline void safely_halt_cpu(int do_minimal_checking)
{
	/* inlining will optimize the branching away */
	if (!do_minimal_checking) {
		if (cpu_data[smp_processor_id()].hlt_works_ok)
			for (;;) {
				halt();
				/* halt failed? still make sure to cpu_relax()! */
				cpu_relax();
			}
		else
			for (;;) {
				cpu_relax();
				cpu_relax();
				cpu_relax();
			}
	} else {
		halt();
		/* halt didn't work, so still keep as cool as possible: */
		for (;;) {
			cpu_relax();
			cpu_relax();
			cpu_relax();
		}
	}
}

Does my preliminary code even make any sense at all? ;)
Might want to cleverly rearrange it to try to get rid of the cpu_relax()
duplication while not abandoning any advantage of those different
conditions.

Andreas Mohr