linux-kernel - Re: [PATCH 1/11] Add generic helpers for arch IPI function calls

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Date:	Thu, 24 Apr 2008 12:59:08 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Mark Lord <lkml@....ca>
Cc:	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	npiggin@...e.de, torvalds@...ux-foundation.org
Subject: Re: [PATCH 1/11] Add generic helpers for arch IPI function calls

On Wed, Apr 23 2008, Mark Lord wrote:
> Jens Axboe wrote:
> >On Wed, Apr 23 2008, Mark Lord wrote:
> >..
> >>The second bug, is that for the halt case at least,
> >>nobody waits for the other CPU to actually halt
> >>before continuing.. so we sometimes enter the shutdown
> >>code while other CPUs are still active.
> >>
> >>This causes some machines to hang at shutdown,
> >>unless CPU_HOTPLUG is configured and takes them offline
> >>before we get here.
> >
> >I'm guessing there's a reason it doesn't pass '1' as the last argument,
> >because that would fix that issue?
> ..
> 
> Undoubtedly -- perhaps the called CPU halts, and therefore cannot reply.  :)

Uhm yes, I guess stop_this_cpu() does exactly what the name implies :-)

> But some kind of pre-halt ack, perhaps plus a short delay by the caller
> after receipt of the ack, would probably suffice to kill that bug.
> 
> But I really haven't studied this code enough to know,
> other than that it historically has been a sticky area
> to poke around in.

Something like this will close the window to right up until the point
where the other CPUs have 'almost' called halt().

diff --git a/arch/x86/kernel/smp.c b/arch/x86/kernel/smp.c
index 5398385..94ec9bf 100644
--- a/arch/x86/kernel/smp.c
+++ b/arch/x86/kernel/smp.c
@@ -155,8 +155,9 @@ static void stop_this_cpu(void *dummy)
 	/*
 	 * Remove this CPU:
 	 */
-	cpu_clear(smp_processor_id(), cpu_online_map);
 	disable_local_APIC();
+	cpu_clear(smp_processor_id(), cpu_online_map);
+	smp_wmb();
 	if (hlt_works(smp_processor_id()))
 		for (;;) halt();
 	for (;;);
@@ -175,6 +176,12 @@ static void native_smp_send_stop(void)
 
 	local_irq_save(flags);
 	smp_call_function(stop_this_cpu, NULL, 0, 0);
+
+	while (cpus_weight(cpu_online_map) > 1) {
+		cpu_relax();
+		smp_rmb();
+	}
+
 	disable_local_APIC();
 	local_irq_restore(flags);
 }

-- 
Jens Axboe

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