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[Xen-devel] [PATCH] xen/arm: fix smpboot barriers



Remove useless smp_wmb() barrier after cpumask_set_cpu(cpuid,
&cpu_online_map), which is not synchronizing against anything.

Keep the other smp_wmb(), before the cpumask_set_cpu call, to ensure
that all writes before setting the cpu online are visible to other cpus.
For that to work properly, we need a corresponding smp_rmb() barrier,
after reading the online cpumask from other processors, which is
currently missing. Add it.

See: http://marc.info/?l=xen-devel&m=148093236307211

Signed-off-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>

diff --git a/xen/arch/arm/smpboot.c b/xen/arch/arm/smpboot.c
index 90ad1d0..c841a15 100644
--- a/xen/arch/arm/smpboot.c
+++ b/xen/arch/arm/smpboot.c
@@ -311,7 +311,6 @@ void start_secondary(unsigned long boot_phys_offset,
 
     /* Now report this CPU is up */
     cpumask_set_cpu(cpuid, &cpu_online_map);
-    smp_wmb();
 
     local_irq_enable();
     local_abort_enable();
@@ -408,6 +407,7 @@ int __cpu_up(unsigned int cpu)
         cpu_relax();
         process_pending_softirqs();
     }
+    smp_rmb();
 
     /*
      * Nuke start of day info before checking one last time if the CPU

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