Re: Re: [PATCH] [RFC] Fix a small window on CPU online/offline

[Keir Fraser <keir.fraser@eu.citrix.com>]

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On 02/04/2010 08:14, "Keir Fraser" <keir.fraser@eu.citrix.com> wrote:

> Hm, actually maybe that could work. You might not even need a flag in case
> current is non-idle in cpu_disable_scheduler(). It might suffice to force
> context_switch() to do full context switch synchronously when the CPU is
> going offline (see appended patch). I was thinking there was a race as soon
> as the cpu is cleared from cpu_online_map, but actually the problem occurs
> only once the vcpu is descheduled, so if we can synchronously sync its state
> before calling context_saved(), perhaps we are fine.
> 
> This could be a very small patch after all! :-)

Perhaps even as small as the attached patch?

It's simple enough we could almost consider it for 4.0.0.

 -- Keir


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diff -r 537451477469 xen/arch/x86/domain.c
--- a/xen/arch/x86/domain.c	Thu Apr 01 09:55:27 2010 +0100
+++ b/xen/arch/x86/domain.c	Fri Apr 02 08:25:30 2010 +0100
@@ -1442,7 +1442,8 @@
 
     set_current(next);
 
-    if ( (per_cpu(curr_vcpu, cpu) == next) || is_idle_vcpu(next) )
+    if ( (per_cpu(curr_vcpu, cpu) == next) ||
+         (is_idle_vcpu(next) && cpu_online(cpu)) )
     {
         local_irq_enable();
     }
diff -r 537451477469 xen/arch/x86/smpboot.c
--- a/xen/arch/x86/smpboot.c	Thu Apr 01 09:55:27 2010 +0100
+++ b/xen/arch/x86/smpboot.c	Fri Apr 02 08:25:30 2010 +0100
@@ -1302,6 +1302,13 @@
 
 	remove_siblinginfo(cpu);
 
+	/*
+	 * If we are running the idle vcpu, sync last non-idle vcpu's state
+	 * before changing cpu_online_map. If we are running non-idle vcpu,
+         * we will synchronously sync the state in context_switch() later.
+	 */
+	__sync_lazy_execstate();
+
 	/* It's now safe to remove this processor from the online map */
 	cpu_clear(cpu, cpu_online_map);
 	fixup_irqs();

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