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@citrix.com>

diff --git a/xen/arch/x86/smpboot.c b/xen/arch/x86/smpboot.c
index 87972146be5209c183d98da04538424ab937f51a..5bf5461ce8f45058ab8f4aa4d94d285c230a3877 100644 (file)
--- a/xen/arch/x86/smpboot.c
+++ b/xen/arch/x86/smpboot.c
@@ -494,6 +494,24 @@ void __devinit start_secondary(void *unused)
                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 *unused)
        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);