tracing/blktrace: Fix to allow setting same value

Masami Hiramatsu reported:

  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

Link: http://lkml.kernel.org/r/[email protected]

Cc: Ingo Molnar <[email protected]>
Cc: Jens Axboe <[email protected]>
Cc: [email protected]
Cc: [email protected]
Fixes: cd649b8 ("blktrace: remove sysfs_blk_trace_enable_show/store()")
Reported-by: Masami Hiramatsu <[email protected]>
Tested-by: Masami Hiramatsu <[email protected]>
Signed-off-by: Steven Rostedt (VMware) <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>

Author: rostedt

kernel/trace/blktrace.c

@@ -1841,6 +1841,10 @@ static ssize_t sysfs_blk_trace_attr_store(struct device *dev,
 	mutex_lock(&q->blk_trace_mutex);
 
 	if (attr == &dev_attr_enable) {
+		if (!!value == !!q->blk_trace) {
+			ret = 0;
+			goto out_unlock_bdev;
+		}
 		if (value)
 			ret = blk_trace_setup_queue(q, bdev);
 		else