[POWERPC] Remove spinlock from struct cpu_purr_data

cpu_purr_data is a per-cpu array used to account for stolen time on
partitioned systems.  It used to be the case that cpus accessed each
others' cpu_purr_data, so each entry was protected by a spinlock.

However, the code was reworked ("Simplify stolen time calculation")
with the result that each cpu accesses its own cpu_purr_data and not
those of other cpus.  This means we can get rid of the spinlock as
long as we're careful to disable interrupts when accessing
cpu_purr_data in process context.
Signed-off-by: NNathan Lynch <ntl@pobox.com>
Signed-off-by: NPaul Mackerras <paulus@samba.org>

arch/powerpc/kernel/time.c

@@ -222,19 +222,28 @@ struct cpu_purr_data {
 	int	initialized;			/* thread is running */
 	u64	tb;			/* last TB value read */
 	u64	purr;			/* last PURR value read */
-	spinlock_t lock;
 };
 
+/*
+ * Each entry in the cpu_purr_data array is manipulated only by its
+ * "owner" cpu -- usually in the timer interrupt but also occasionally
+ * in process context for cpu online.  As long as cpus do not touch
+ * each others' cpu_purr_data, disabling local interrupts is
+ * sufficient to serialize accesses.
+ */
 static DEFINE_PER_CPU(struct cpu_purr_data, cpu_purr_data);
 
 static void snapshot_tb_and_purr(void *data)
 {
+	unsigned long flags;
 	struct cpu_purr_data *p = &__get_cpu_var(cpu_purr_data);
 
+	local_irq_save(flags);
 	p->tb = mftb();
 	p->purr = mfspr(SPRN_PURR);
 	wmb();
 	p->initialized = 1;
+	local_irq_restore(flags);
 }
 
 /*
@@ -242,15 +251,14 @@ static void snapshot_tb_and_purr(void *data)
  */
 void snapshot_timebases(void)
 {
-	int cpu;
-
 	if (!cpu_has_feature(CPU_FTR_PURR))
 		return;
-	for_each_possible_cpu(cpu)
-		spin_lock_init(&per_cpu(cpu_purr_data, cpu).lock);
 	on_each_cpu(snapshot_tb_and_purr, NULL, 0, 1);
 }
 
+/*
+ * Must be called with interrupts disabled.
+ */
 void calculate_steal_time(void)
 {
 	u64 tb, purr;
@@ -262,7 +270,6 @@ void calculate_steal_time(void)
 	pme = &per_cpu(cpu_purr_data, smp_processor_id());
 	if (!pme->initialized)
 		return;		/* this can happen in early boot */
-	spin_lock(&pme->lock);
 	tb = mftb();
 	purr = mfspr(SPRN_PURR);
 	stolen = (tb - pme->tb) - (purr - pme->purr);
@@ -270,7 +277,6 @@ void calculate_steal_time(void)
 		account_steal_time(current, stolen);
 	pme->tb = tb;
 	pme->purr = purr;
-	spin_unlock(&pme->lock);
 }
 
 /*
@@ -284,12 +290,12 @@ static void snapshot_purr(void)
 
 	if (!cpu_has_feature(CPU_FTR_PURR))
 		return;
+	local_irq_save(flags);
 	pme = &per_cpu(cpu_purr_data, smp_processor_id());
-	spin_lock_irqsave(&pme->lock, flags);
 	pme->tb = mftb();
 	pme->purr = mfspr(SPRN_PURR);
 	pme->initialized = 1;
-	spin_unlock_irqrestore(&pme->lock, flags);
+	local_irq_restore(flags);
 }
 
 #endif /* CONFIG_PPC_SPLPAR */