[Xen-changelog] [xen-unstable] x86: Disable spinlock checks temporarily while bringing a CPU online.

[Xen patchbot-unstable <patchbot-unstable@xxxxxxxxxxxxxxxxxxx>]

# HG changeset patch
# User Keir Fraser <keir.fraser@xxxxxxxxxx>
# Date 1258040077 0
# Node ID 32a27fe01e07b55a9390a575139f4d8a2d6fdbea
# Parent  accded2f185f4178f875b170a5c01544648a68d2
x86: Disable spinlock checks temporarily while bringing a CPU online.

This is safe, as described in a code comment. Also fix up another
comment in start_secondary() while we're there.

Signed-off-by: Keir Fraser <keir.fraser@xxxxxxxxxx>
---
 xen/arch/x86/smpboot.c |   25 ++++++++++++++++++++++++-
 1 files changed, 24 insertions(+), 1 deletion(-)

diff -r accded2f185f -r 32a27fe01e07 xen/arch/x86/smpboot.c
--- a/xen/arch/x86/smpboot.c    Thu Nov 12 13:15:40 2009 +0000
+++ b/xen/arch/x86/smpboot.c    Thu Nov 12 15:34:37 2009 +0000
@@ -494,6 +494,24 @@ void __devinit start_secondary(void *unu
                rdmsrl(MSR_EFER, this_cpu(efer));
        asm volatile ( "mov %%cr4,%0" : "=r" (this_cpu(cr4)) );
 
+       /*
+        * Just as during early bootstrap, it is convenient here to disable
+        * spinlock checking while we have IRQs disabled. This allows us to
+        * acquire IRQ-unsafe locks when it would otherwise be disallowed.
+        * 
+        * It is safe because the race we are usually trying to avoid involves
+        * a group of CPUs rendezvousing in an IPI handler, where one cannot
+        * join because it is spinning with IRQs disabled waiting to acquire a
+        * lock held by another in the rendezvous group (the lock must be an
+        * IRQ-unsafe lock since the CPU took the IPI after acquiring it, and
+        * hence had IRQs enabled). This is a deadlock scenario.
+        * 
+        * However, no CPU can be involved in rendezvous until it is online,
+        * hence no such group can be waiting for this CPU until it is
+        * visible in cpu_online_map. Hence such a deadlock is not possible.
+        */
+       spin_debug_disable();
+
        percpu_traps_init();
 
        cpu_init();
@@ -517,10 +535,15 @@ void __devinit start_secondary(void *unu
        flush_tlb_local();
 
        /* This must be done before setting cpu_online_map */
+       spin_debug_enable();
        set_cpu_sibling_map(raw_smp_processor_id());
        wmb();
 
-       /* Initlize vector_irq for BSPs */
+       /*
+        * We need to hold vector_lock so there the set of online cpus
+        * does not change while we are assigning vectors to cpus.  Holding
+        * this lock ensures we don't half assign or remove an irq from a cpu.
+        */
        lock_vector_lock();
        __setup_vector_irq(smp_processor_id());
        cpu_set(smp_processor_id(), cpu_online_map);

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