Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Tue, 16 Aug 2022 13:48:43 -0700
From:   Krister Johansen <kjlx@templeofstupid.com>
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     Ingo Molnar <mingo@redhat.com>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        David Reaver <me@davidreaver.com>,
        linux-kernel@vger.kernel.org, Jiri Olsa <jolsa@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Frederic Weisbecker <fweisbec@gmail.com>
Subject: Re: [PATCH 1/1] tracing: fix a WARN from trace_event_dyn_put_ref
Message-ID: <20220816204843.GB2004@templeofstupid.com>
References: <cover.1660347763.git.kjlx@templeofstupid.com>
 <4e43a4eece5f382d1636397fb3c0208f2afe81fc.1660347763.git.kjlx@templeofstupid.com>
 <20220816122559.17869abc@gandalf.local.home>
 <20220816185418.GA2004@templeofstupid.com>
 <20220816152411.086ce631@gandalf.local.home>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20220816152411.086ce631@gandalf.local.home>
Precedence: bulk

On Tue, Aug 16, 2022 at 03:24:11PM -0400, Steven Rostedt wrote:
> Actually, I think the issue is that the unreg() calls trace_event_put_ref()
> but the reg() does not do the try_get_ref(), but the perf_trace_init()
> does. Which is the broken symmetry.
> 
> I think if we pull out the trace_event_put_ref() from the unreg() function,
> we fix the bug and also put back the symmetry.
> 
> Does this patch work?

Yes, thanks, this change looks correct.  It also does build and pass the
tests that I included in the original e-mail.

> diff --git a/kernel/trace/trace_event_perf.c b/kernel/trace/trace_event_perf.c
> index a114549720d6..61e3a2620fa3 100644
> --- a/kernel/trace/trace_event_perf.c
> +++ b/kernel/trace/trace_event_perf.c
> @@ -157,7 +157,7 @@ static void perf_trace_event_unreg(struct perf_event *p_event)
>  	int i;
>  
>  	if (--tp_event->perf_refcount > 0)
> -		goto out;
> +		return;
>  
>  	tp_event->class->reg(tp_event, TRACE_REG_PERF_UNREGISTER, NULL);
>  
> @@ -176,8 +176,6 @@ static void perf_trace_event_unreg(struct perf_event *p_event)
>  			perf_trace_buf[i] = NULL;
>  		}
>  	}
> -out:
> -	trace_event_put_ref(tp_event);
>  }
>  
>  static int perf_trace_event_open(struct perf_event *p_event)
> @@ -241,6 +239,7 @@ void perf_trace_destroy(struct perf_event *p_event)
>  	mutex_lock(&event_mutex);
>  	perf_trace_event_close(p_event);
>  	perf_trace_event_unreg(p_event);
> +	trace_event_put_ref(p_event->tp_event);
>  	mutex_unlock(&event_mutex);
>  }
>  
> @@ -292,6 +291,7 @@ void perf_kprobe_destroy(struct perf_event *p_event)
>  	mutex_lock(&event_mutex);
>  	perf_trace_event_close(p_event);
>  	perf_trace_event_unreg(p_event);
> +	trace_event_put_ref(p_event->tp_event);
>  	mutex_unlock(&event_mutex);
>  
>  	destroy_local_trace_kprobe(p_event->tp_event);
> @@ -347,6 +347,7 @@ void perf_uprobe_destroy(struct perf_event *p_event)
>  	mutex_lock(&event_mutex);
>  	perf_trace_event_close(p_event);
>  	perf_trace_event_unreg(p_event);
> +	trace_event_put_ref(p_event->tp_event);
>  	mutex_unlock(&event_mutex);
>  	destroy_local_trace_uprobe(p_event->tp_event);
>  }
> 

If it matters:

Reviewed-by: Krister Johansen <kjlx@templeofstupid.com>
Tested-by: Krister Johansen <kjlx@templeofstupid.com>

Thanks again,

-K