[PATCH 3.16 146/328] tracing/blktrace: Fix to allow setting same value

Ben Hutchings
2018-12-09T22:14:08Z

From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>

commit 757d9140072054528b13bbe291583d9823cde195 upstream.

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/20180816103802.08678002@gandalf.local.home

Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: linux-block@vger.kernel.org
Fixes: cd649b8bb830d ("blktrace: remove sysfs_blk_trace_enable_show/store()")
Reported-by: Masami Hiramatsu <mhiramat@kernel.org>
Tested-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
kernel/trace/blktrace.c | 4 ++++
1 file changed, 4 insertions(+)

--- a/kernel/trace/blktrace.c
+++ b/kernel/trace/blktrace.c
@@ -1778,6 +1778,10 @@ static ssize_t sysfs_blk_trace_attr_stor
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