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Date:   Fri, 31 Jul 2020 13:33:45 -0600
From:   dann frazier <dann.frazier@canonical.com>
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     linux-kernel@vger.kernel.org, Ingo Molnar <mingo@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Ian May <ian.may@canonical.com>
Subject: Re: [for-next][PATCH 09/18] tracing: Move pipe reference to trace
 array instead of current_tracer
Message-ID: <20200731193345.GA54749@xps13.dannf>
References: <20200702215812.428188663@goodmis.org>
 <20200702215832.784471709@goodmis.org>
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On Thu, Jul 02, 2020 at 05:58:21PM -0400, Steven Rostedt wrote:
> From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>
> 
> If a process has the trace_pipe open on a trace_array, the current tracer
> for that trace array should not be changed. This was original enforced by a
> global lock, but when instances were introduced, it was moved to the
> current_trace. But this structure is shared by all instances, and a
> trace_pipe is for a single instance. There's no reason that a process that
> has trace_pipe open on one instance should prevent another instance from
> changing its current tracer. Move the reference counter to the trace_array
> instead.
> 
> This is marked as "Fixes" but is more of a clean up than a true fix.
> Backport if you want, but its not critical.

Thanks Steven! In case it helps backport consideration, I wanted to
note that this addresses an issue we've seen with users trying to
change current_tracer when they happen to have rasdaemon
installed. rasdaemon uses the trace_pipe interface at runtime, which
therefore blocks changing the current tracer. But of course, unless
you know about rasdaemon internals, it isn't exactly an obvious
failure mode.

  -dann

> Fixes: cf6ab6d9143b1 ("tracing: Add ref count to tracer for when they are being read by pipe")
> Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
> ---
>  kernel/trace/trace.c | 12 ++++++------
>  kernel/trace/trace.h |  2 +-
>  2 files changed, 7 insertions(+), 7 deletions(-)
> 
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index 8241d1448d70..64c5b8146cca 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -5891,7 +5891,7 @@ int tracing_set_tracer(struct trace_array *tr, const char *buf)
>  	}
>  
>  	/* If trace pipe files are being read, we can't change the tracer */
> -	if (tr->current_trace->ref) {
> +	if (tr->trace_ref) {
>  		ret = -EBUSY;
>  		goto out;
>  	}
> @@ -6107,7 +6107,7 @@ static int tracing_open_pipe(struct inode *inode, struct file *filp)
>  
>  	nonseekable_open(inode, filp);
>  
> -	tr->current_trace->ref++;
> +	tr->trace_ref++;
>  out:
>  	mutex_unlock(&trace_types_lock);
>  	return ret;
> @@ -6126,7 +6126,7 @@ static int tracing_release_pipe(struct inode *inode, struct file *file)
>  
>  	mutex_lock(&trace_types_lock);
>  
> -	tr->current_trace->ref--;
> +	tr->trace_ref--;
>  
>  	if (iter->trace->pipe_close)
>  		iter->trace->pipe_close(iter);
> @@ -7428,7 +7428,7 @@ static int tracing_buffers_open(struct inode *inode, struct file *filp)
>  
>  	filp->private_data = info;
>  
> -	tr->current_trace->ref++;
> +	tr->trace_ref++;
>  
>  	mutex_unlock(&trace_types_lock);
>  
> @@ -7529,7 +7529,7 @@ static int tracing_buffers_release(struct inode *inode, struct file *file)
>  
>  	mutex_lock(&trace_types_lock);
>  
> -	iter->tr->current_trace->ref--;
> +	iter->tr->trace_ref--;
>  
>  	__trace_array_put(iter->tr);
>  
> @@ -8737,7 +8737,7 @@ static int __remove_instance(struct trace_array *tr)
>  	int i;
>  
>  	/* Reference counter for a newly created trace array = 1. */
> -	if (tr->ref > 1 || (tr->current_trace && tr->current_trace->ref))
> +	if (tr->ref > 1 || (tr->current_trace && tr->trace_ref))
>  		return -EBUSY;
>  
>  	list_del(&tr->list);
> diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
> index 13db4000af3f..f21607f87189 100644
> --- a/kernel/trace/trace.h
> +++ b/kernel/trace/trace.h
> @@ -356,6 +356,7 @@ struct trace_array {
>  	struct trace_event_file *trace_marker_file;
>  	cpumask_var_t		tracing_cpumask; /* only trace on set CPUs */
>  	int			ref;
> +	int			trace_ref;
>  #ifdef CONFIG_FUNCTION_TRACER
>  	struct ftrace_ops	*ops;
>  	struct trace_pid_list	__rcu *function_pids;
> @@ -547,7 +548,6 @@ struct tracer {
>  	struct tracer		*next;
>  	struct tracer_flags	*flags;
>  	int			enabled;
> -	int			ref;
>  	bool			print_max;
>  	bool			allow_instances;
>  #ifdef CONFIG_TRACER_MAX_TRACE