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

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Date:   Thu, 16 Aug 2018 12:49:17 -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 Fri, 17 Aug 2018 01:38:47 +0900
Masami Hiramatsu <mhiramat@...nel.org> wrote:

> On Thu, 16 Aug 2018 10:38:02 -0400
> Steven Rostedt <rostedt@...dmis.org> wrote:
> 
> > 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?  
> 
> Ah, yes, but we need to clear "ret" in that case.

Sure.

> 
> Thanks!
> 
> > 
> > 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) {
    +                   ret = 0;
> > +			goto out_unlock_bdev;
    +		}

-- Steve

> >  		if (value)
> >  			ret = blk_trace_setup_queue(q, bdev);
> >  		else
> > 
> > -- Steve  
> 
>