linux-kernel - Re: [PATCH 2/2] tracing/blktrace: Fix to allow setting same value

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Date:   Thu, 16 Aug 2018 10:38:02 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org
Subject: Re: [PATCH 2/2] tracing/blktrace: Fix to allow setting same value

On Thu, 16 Aug 2018 18:51:15 +0900
Masami Hiramatsu <mhiramat@...nel.org> wrote:

> 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@...nel.org>
> Cc: Jens Axboe <axboe@...nel.dk>
> Cc: linux-block@...r.kernel.org
> ---
>  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;

Hmm, why not just do this? Does this fix it too?

diff --git a/kernel/trace/blktrace.c b/kernel/trace/blktrace.c
index 987d9a9ae283..10b2b9c5fd25 100644
--- a/kernel/trace/blktrace.c
+++ b/kernel/trace/blktrace.c
@@ -1841,6 +1841,8 @@ 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)
+			goto out_unlock_bdev;
 		if (value)
 			ret = blk_trace_setup_queue(q, bdev);
 		else

-- Steve