[PATCH 2/2] tracing/blktrace: Fix to allow setting same value

[Masami Hiramatsu <mhiramat@xxxxxxxxxx>]

Current trace-enable attribute in sysfs returns an error
if user writes the same setting value as current one,
e.g.

  # cat /sys/block/sda/trace/enable
  0
  # echo 0 > /sys/block/sda/trace/enable
  bash: echo: write error: Invalid argument
  # echo 1 > /sys/block/sda/trace/enable
  # echo 1 > /sys/block/sda/trace/enable
  bash: echo: write error: Device or resource busy

But this is not a preferred behavior, it should ignore
if new setting is same as current one. This fixes the
problem as below.

  # cat /sys/block/sda/trace/enable
  0
  # echo 0 > /sys/block/sda/trace/enable
  # echo 1 > /sys/block/sda/trace/enable
  # echo 1 > /sys/block/sda/trace/enable

Signed-off-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
Cc: Jens Axboe <axboe@xxxxxxxxx>
Cc: linux-block@xxxxxxxxxxxxxxx
---
 kernel/trace/blktrace.c |   11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/kernel/trace/blktrace.c b/kernel/trace/blktrace.c
index 987d9a9ae283..e67db1e8a000 100644
--- a/kernel/trace/blktrace.c
+++ b/kernel/trace/blktrace.c
@@ -1602,11 +1602,11 @@ static int blk_trace_remove_queue(struct request_queue *q)
 	struct blk_trace *bt;
 
 	bt = xchg(&q->blk_trace, NULL);
-	if (bt == NULL)
-		return -EINVAL;
+	if (bt != NULL) {
+		put_probe_ref();
+		blk_trace_free(bt);
+	}
 
-	put_probe_ref();
-	blk_trace_free(bt);
 	return 0;
 }
 
@@ -1619,6 +1619,9 @@ static int blk_trace_setup_queue(struct request_queue *q,
 	struct blk_trace *bt = NULL;
 	int ret = -ENOMEM;
 
+	if (q->blk_trace)
+		return 0;
+
 	bt = kzalloc(sizeof(*bt), GFP_KERNEL);
 	if (!bt)
 		return -ENOMEM;