perf_events: Fix perf buffer watermark setting

Since 2.6.36 (specifically commit d57e34fdd60b ("perf: Simplify the
ring-buffer logic: make perf_buffer_alloc() do everything needed"),
the perf_buffer_init_code() has been mis-setting the buffer watermark
if perf_event_attr.wakeup_events has a non-zero value.

This is because perf_event_attr.wakeup_events is a union with
perf_event_attr.wakeup_watermark.

This commit re-enables the check for perf_event_attr.watermark being
set before continuing with setting a non-default watermark.

This bug is most noticable when you are trying to use PERF_IOC_REFRESH
with a value larger than one and perf_event_attr.wakeup_events is set to
one.  In this case the buffer watermark will be set to 1 and you will
get extraneous POLL_IN overflows rather than POLL_HUP as expected.

[ avoid using attr.wakeup_events when attr.watermark is set ]

Signed-off-by: Vince Weaver <vweaver1@eecs.utk.edu>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: <stable@kernel.org>
Link: http://lkml.kernel.org/r/alpine.DEB.2.00.1106011506390.5384@cl320.eecs.utk.edu
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/events/core.c

@@ -3569,8 +3569,10 @@ static int perf_mmap(struct file *file, struct vm_area_struct *vma)
 	if (vma->vm_flags & VM_WRITE)
 		flags |= RING_BUFFER_WRITABLE;
 
-	rb = rb_alloc(nr_pages, event->attr.wakeup_watermark,
-				   event->cpu, flags);
+	rb = rb_alloc(nr_pages, 
+		event->attr.watermark ? event->attr.wakeup_watermark : 0,
+		event->cpu, flags);
+
 	if (!rb) {
 		ret = -ENOMEM;
 		goto unlock;

kernel/events/ring_buffer.c

@@ -199,13 +199,15 @@ void perf_output_end(struct perf_output_handle *handle)
 	struct perf_event *event = handle->event;
 	struct ring_buffer *rb = handle->rb;
 
-	int wakeup_events = event->attr.wakeup_events;
-
-	if (handle->sample && wakeup_events) {
-		int events = local_inc_return(&rb->events);
-		if (events >= wakeup_events) {
-			local_sub(wakeup_events, &rb->events);
-			local_inc(&rb->wakeup);
+	if (handle->sample && !event->attr.watermark) {
+		int wakeup_events = event->attr.wakeup_events;
+
+		if (wakeup_events) {
+			int events = local_inc_return(&rb->events);
+			if (events >= wakeup_events) {
+				local_sub(wakeup_events, &rb->events);
+				local_inc(&rb->wakeup);
+			}
 		}
 	}