[PATCH 5/5] percpu_counter: only disable preemption if needed in add_unless_lt()

[Alex Elder]

In __percpu_counter_add_unless_lt() we don't need to disable
preemption unless we're manipulating a per-cpu variable.  That only
happens in a limited case, so narrow the scope of that preemption to
surround that case.  This makes the "out" label rather unnecessary,
so replace a couple "goto out" calls to just return.

Signed-off-by: Alex Elder <aelder at sgi.com>

---
 lib/percpu_counter.c |   21 ++++++++++-----------
 1 file changed, 10 insertions(+), 11 deletions(-)

Index: b/lib/percpu_counter.c
===================================================================
--- a/lib/percpu_counter.c
+++ b/lib/percpu_counter.c
@@ -232,8 +232,6 @@ int __percpu_counter_add_unless_lt(struc
 	int	cpu;
 	int	ret = -1;
 
-	preempt_disable();
-
 	/*
 	 * Check to see if rough count will be sufficient for
 	 * comparison.  First, if the upper bound is too low,
@@ -241,7 +239,7 @@ int __percpu_counter_add_unless_lt(struc
 	 */
 	count = percpu_counter_read(fbc);
 	if (count + error + amount < threshold)
-		goto out;
+		return -1;
 
 	/*
 	 * Next, if the lower bound is above the threshold, we can
@@ -251,12 +249,15 @@ int __percpu_counter_add_unless_lt(struc
 	if (count - error + amount > threshold) {
 		s32 *pcount = this_cpu_ptr(fbc->counters);
 
+		preempt_disable();
+		pcount = this_cpu_ptr(fbc->counters);
 		count = *pcount + amount;
 		if (abs(count) < batch) {
 			*pcount = count;
-			ret = 1;
-			goto out;
+			preempt_enable();
+			return 1;
 		}
+		preempt_enable();
 	}
 
 	/*
@@ -281,10 +282,9 @@ int __percpu_counter_add_unless_lt(struc
 	}
 
 	/*
-	 * Result is withing the error margin. Run an open-coded sum of the
-	 * per-cpu counters to get the exact value at this point in time,
-	 * and if the result greater than the threshold, add the amount to
-	 * the global counter.
+	 * Now add in all the per-cpu counters to compute the exact
+	 * value at this point in time, and if the result greater
+	 * than the threshold, add the amount to the global counter.
 	 */
 	count = fbc->count;
 	for_each_online_cpu(cpu) {
@@ -301,8 +301,7 @@ int __percpu_counter_add_unless_lt(struc
 	}
 out_unlock:
 	spin_unlock(&fbc->lock);
-out:
-	preempt_enable();
+
 	return ret;
 }
 EXPORT_SYMBOL(__percpu_counter_add_unless_lt);