linux-kernel - Re: [PATCH v5 3/4] tracing: Remove the backup instance automatically after read

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Message-Id: <20260130182409.3e5b9f86dcfe7625b3b582cd@kernel.org>
Date: Fri, 30 Jan 2026 18:24:09 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
 linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH v5 3/4] tracing: Remove the backup instance
 automatically after read

On Thu, 29 Jan 2026 15:13:52 -0500
Steven Rostedt <rostedt@...dmis.org> wrote:

> On Wed, 28 Jan 2026 09:10:04 +0900
> "Masami Hiramatsu (Google)" <mhiramat@...nel.org> wrote:
> 
> > From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
> > 
> > Since the backup instance is readonly, after reading all data
> > via pipe, no data is left on the instance. Thus it can be
> > removed safely after closing all files.
> > This also removes it if user resets the ring buffer manually
> > via 'trace' file.
> > 
> > Signed-off-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>
> > ---
> >  Changes in v4:
> >    - Update description.
> > ---
> >  kernel/trace/trace.c |   64 +++++++++++++++++++++++++++++++++++++++++++++++++-
> >  kernel/trace/trace.h |    6 +++++
> >  2 files changed, 69 insertions(+), 1 deletion(-)
> > 
> > diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> > index d39f6509c12a..7d615a74f915 100644
> > --- a/kernel/trace/trace.c
> > +++ b/kernel/trace/trace.c
> > @@ -590,6 +590,55 @@ void trace_set_ring_buffer_expanded(struct trace_array *tr)
> >  	tr->ring_buffer_expanded = true;
> >  }
> >  
> > +static int __remove_instance(struct trace_array *tr);
> > +
> > +static void trace_array_autoremove(struct work_struct *work)
> > +{
> > +	struct trace_array *tr = container_of(work, struct trace_array, autoremove_work);
> > +
> > +	guard(mutex)(&event_mutex);
> > +	guard(mutex)(&trace_types_lock);
> > +
> > +	/*
> > +	 * This can be fail if someone gets @tr before starting this
> > +	 * function, but in that case, this will be kicked again when
> > +	 * putting it. So we don't care the result.
> 
> 			"So we don't care about the result."

OK.

> 
> > +	 */
> > +	__remove_instance(tr);
> > +}
> > +
> > +static struct workqueue_struct *autoremove_wq;
> > +
> > +static void trace_array_init_autoremove(struct trace_array *tr)
> > +{
> > +	INIT_WORK(&tr->autoremove_work, trace_array_autoremove);
> > +}
> > +
> > +static void trace_array_kick_autoremove(struct trace_array *tr)
> > +{
> > +	if (!work_pending(&tr->autoremove_work) && autoremove_wq)
> > +		queue_work(autoremove_wq, &tr->autoremove_work);
> > +}
> > +
> > +static void trace_array_cancel_autoremove(struct trace_array *tr)
> > +{
> > +	if (work_pending(&tr->autoremove_work))
> > +		cancel_work(&tr->autoremove_work);
> > +}
> > +
> > +__init static int trace_array_init_autoremove_wq(void)
> > +{
> 
> This isn't needed if there's no backup trace_array right?
> 
> Instead of creating a work queue when its not needed, just exit out if
> there's no backup trace_array.
> 
> Oh, and the above functions should always test autoremove_wq for NULL.

Ah, right. OK, let me fix it.

> 
> > +	autoremove_wq = alloc_workqueue("tr_autoremove_wq",
> > +					WQ_UNBOUND | WQ_HIGHPRI, 0);
> > +	if (!autoremove_wq) {
> > +		pr_err("Unable to allocate tr_autoremove_wq\n");
> > +		return -ENOMEM;
> > +	}
> > +	return 0;
> > +}
> > +
> > +late_initcall_sync(trace_array_init_autoremove_wq);
> > +
> >  LIST_HEAD(ftrace_trace_arrays);
> >  
> >  int trace_array_get(struct trace_array *this_tr)
> > @@ -598,7 +647,7 @@ int trace_array_get(struct trace_array *this_tr)
> >  
> >  	guard(mutex)(&trace_types_lock);
> >  	list_for_each_entry(tr, &ftrace_trace_arrays, list) {
> > -		if (tr == this_tr) {
> > +		if (tr == this_tr && !tr->free_on_close) {
> >  			tr->ref++;
> >  			return 0;
> >  		}
> 
> Break the above into:
> 
> 		if (tr == this_tr) {
> 			if (tr->free_on_close)
> 				break;
> 			tr->ref++;
> 			return 0;
> 		}
> 
> Why continue the loop if we found the trace_array but it's in the process
> of closing?

Ah, indeed. OK, I'll fix it.

Thank you!

> 
> -- Steve
> 


-- 
Masami Hiramatsu (Google) <mhiramat@...nel.org>