1 files changed, 38 insertions, 4 deletions

diff --git a/arch/arm/kernel/smp.c b/arch/arm/kernel/smp.c
index 4619177bcfe6..47ab90563bf4 100644
--- a/arch/arm/kernel/smp.c
+++ b/arch/arm/kernel/smp.c
@@ -211,6 +211,13 @@ void __cpuinit __cpu_die(unsigned int cpu)
         }
         printk(KERN_NOTICE "CPU%u: shutdown\n", cpu);
 
+        /*
+         * platform_cpu_kill() is generally expected to do the powering off
+         * and/or cutting of clocks to the dying CPU.  Optionally, this may
+         * be done by the CPU which is dying in preference to supporting
+         * this call, but that means there is _no_ synchronisation between
+         * the requesting CPU and the dying CPU actually losing power.
+         */
         if (!platform_cpu_kill(cpu))
                 printk("CPU%u: unable to kill\n", cpu);
 }
@@ -230,14 +237,41 @@ void __ref cpu_die(void)
         idle_task_exit();
 
         local_irq_disable();
-        mb();
 
-        /* Tell __cpu_die() that this CPU is now safe to dispose of */
+        /*
+         * Flush the data out of the L1 cache for this CPU.  This must be
+         * before the completion to ensure that data is safely written out
+         * before platform_cpu_kill() gets called - which may disable
+         * *this* CPU and power down its cache.
+         */
+        flush_cache_louis();
+
+        /*
+         * Tell __cpu_die() that this CPU is now safe to dispose of.  Once
+         * this returns, power and/or clocks can be removed at any point
+         * from this CPU and its cache by platform_cpu_kill().
+         */
         RCU_NONIDLE(complete(&cpu_died));
 
         /*
-         * actual CPU shutdown procedure is at least platform (if not
-         * CPU) specific.
+         * Ensure that the cache lines associated with that completion are
+         * written out.  This covers the case where _this_ CPU is doing the
+         * powering down, to ensure that the completion is visible to the
+         * CPU waiting for this one.
+         */
+        flush_cache_louis();
+
+        /*
+         * The actual CPU shutdown procedure is at least platform (if not
+         * CPU) specific.  This may remove power, or it may simply spin.
+         *
+         * Platforms are generally expected *NOT* to return from this call,
+         * although there are some which do because they have no way to
+         * power down the CPU.  These platforms are the _only_ reason we
+         * have a return path which uses the fragment of assembly below.
+         *
+         * The return path should not be used for platforms which can
+         * power off the CPU.
          */
         if (smp_ops.cpu_die)
                 smp_ops.cpu_die(cpu);