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Date:	Mon, 10 Nov 2014 11:40:32 +0100
From:	Arianna Avanzini <avanzini.arianna@...il.com>
To:	Namhyung Kim <namhyung@...nel.org>
Cc:	rostedt@...dmis.org, mingo@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC] trace, blktrace: remove trace from running list only
 if trace is running

On Mon, Nov 10, 2014 at 05:25:56PM +0900, Namhyung Kim wrote:
> Hi Arianna,

Hi Namhyung,

thank you for replying.

> 
> On Sat,  8 Nov 2014 16:14:40 +0100, Arianna Avanzini wrote:
> > Currently, blktrace can be started/stopped via its ioctl-based interface
> > (used by the userspace blktrace tool) or via its ftrace interface. The
> > function blk_trace_remove_queue(), called each time an "enable" tunable
> > of the ftrace interface transitions to zero, removes unconditionally the
> > trace from the running list, even if its state is not Blktrace_running.
> > In fact, the state of a blk_trace is modified only by the ioctl-based
> > interface, and a blk_trace is added to the running list only when its
> > state transitions from Blktrace_setup or Blktrace_stopped to
> > Blktrace_running. If the ioctl-based interface is not being used, the
> > state of the blk_trace is undefined.
> > In this case, using the sysfs tunable to stop a trace would trigger a
> > removal of a blk_trace from the running list while it is not on such a
> > list, leading to a null pointer dereference. This commit attempts to fix
> > the issue by letting the blk_trace_remove_queue() function remove the
> > blk_trace from the running list only if its state is Blktrace_running.
> 
> What about just getting rid of the list_del()?  blk_trace_setup_queue()
> doesn't add it to running_trace_list and I think we should prevent mix
> of ioctl and sysfs usage somehow..
> 

And blk_trace_remove_queue() is used only by the sysfs interface, you're
right. I'm re-sending the patch with your comment applied.

Thank you,
Arianna


> Thanks,
> Namhyung
> 
> 
> >
> > Signed-off-by: Arianna Avanzini <avanzini.arianna@...il.com>
> > ---
> >  kernel/trace/blktrace.c | 8 +++++---
> >  1 file changed, 5 insertions(+), 3 deletions(-)
> >
> > diff --git a/kernel/trace/blktrace.c b/kernel/trace/blktrace.c
> > index c1bd4ad..f58b617 100644
> > --- a/kernel/trace/blktrace.c
> > +++ b/kernel/trace/blktrace.c
> > @@ -1493,9 +1493,11 @@ static int blk_trace_remove_queue(struct request_queue *q)
> >  	if (atomic_dec_and_test(&blk_probes_ref))
> >  		blk_unregister_tracepoints();
> >  
> > -	spin_lock_irq(&running_trace_lock);
> > -	list_del(&bt->running_list);
> > -	spin_unlock_irq(&running_trace_lock);
> > +	if (bt->trace_state == Blktrace_running) {
> > +		spin_lock_irq(&running_trace_lock);
> > +		list_del(&bt->running_list);
> > +		spin_unlock_irq(&running_trace_lock);
> > +	}
> >  	blk_trace_free(bt);
> >  	return 0;
> >  }

-- 
/*
 * Arianna Avanzini
 * avanzini.arianna@...il.com
 * http://ava.webhop.me
 */
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