perf mmap: Discard head in overwrite_rb_find_range()

In overwrite mode, start will be set to head in perf_mmap__read_init().
Therefore, there is no need to set the start one more time in
overwrite_rb_find_range() and *start can be used as head instead of
passing head to overwrite_rb_find_range().

Signed-off-by: Yisheng Xie <[e-mail]>
Reviewed-by: Kan Liang <[e-mail]>
Cc: Alexander Shishkin <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/util/mmap.c

···
 	return 0;
 }
 
-static int overwrite_rb_find_range(void *buf, int mask, u64 head, u64 *start, u64 *end)
+static int overwrite_rb_find_range(void *buf, int mask, u64 *start, u64 *end)
 {
 	struct perf_event_header *pheader;
-	u64 evt_head = head;
+	u64 evt_head = *start;
 	int size = mask + 1;
 
-	pr_debug2("overwrite_rb_find_range: buf=%p, head=%"PRIx64"\n", buf, head);
-	pheader = (struct perf_event_header *)(buf + (head & mask));
-	*start = head;
+	pr_debug2("%s: buf=%p, start=%"PRIx64"\n", __func__, buf, *start);
+	pheader = (struct perf_event_header *)(buf + (*start & mask));
 	while (true) {
-		if (evt_head - head >= (unsigned int)size) {
+		if (evt_head - *start >= (unsigned int)size) {
 			pr_debug("Finished reading overwrite ring buffer: rewind\n");
-			if (evt_head - head > (unsigned int)size)
+			if (evt_head - *start > (unsigned int)size)
 				evt_head -= pheader->size;
 			*end = evt_head;
 			return 0;
···
 		 * Backward ring buffer is full. We still have a chance to read
 		 * most of data from it.
 		 */
-		if (overwrite_rb_find_range(data, md->mask, head, &md->start, &md->end))
+		if (overwrite_rb_find_range(data, md->mask, &md->start, &md->end))
 			return -EINVAL;
 	}