linux-kernel - Re: ftrace/perf

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Date:	Wed, 01 Sep 2010 11:04:28 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Avi Kivity <avi@...hat.com>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	kvm-devel <kvm@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: ftrace/perf_event leak

On Wed, 2010-09-01 at 11:42 +0300, Avi Kivity wrote:
> I recently added perf_event support to kvm_stat, to display kvm 
> tracepoints as statistics (I'd like to fold this to tools/perf 
> eventually, but that's another story).  However I'm seeing a resource 
> leak - after I quit the tool, there are quite a few references into the 
> kvm module:
> 
>    kvm_intel              43655  0
>    kvm                   272984  269 kvm_intel
> 
> The tool is just a python script that reads 
> /sys/kernel/debug/tracing/events/kvm to find out which events are 
> available, uses perf_event_open() to create one group per cpu to which a 
> lot of events are attached.  The only special thing I can think of is 
> that we use an ioctl to attach a filter to many perf_event descriptors.
> 
> You can find the source at 
> http://git.kernel.org/?p=virt/kvm/qemu-kvm.git;a=blob_plain;f=kvm/kvm_stat;hb=5bd5f131b50cb373ff4e2a3632c6dad00a1f0b55.  
> All it needs are the kvm modules loaded; no need to actually run a 
> guest.  Run as root.
> 

Does something like the below cure that?

I seem to remember C doesn't make any promises about the order of logic
statements, hence we need to explicitly pull out that try_module_get()
so that it evaluates after the rest of the conditions.

---
 kernel/trace/trace_event_perf.c |   10 ++++++----
 1 files changed, 6 insertions(+), 4 deletions(-)

diff --git a/kernel/trace/trace_event_perf.c b/kernel/trace/trace_event_perf.c
index 000e6e8..35051f2 100644
--- a/kernel/trace/trace_event_perf.c
+++ b/kernel/trace/trace_event_perf.c
@@ -88,10 +88,11 @@ int perf_trace_init(struct perf_event *p_event)
 	mutex_lock(&event_mutex);
 	list_for_each_entry(tp_event, &ftrace_events, list) {
 		if (tp_event->event.type == event_id &&
-		    tp_event->class && tp_event->class->reg &&
-		    try_module_get(tp_event->mod)) {
-			ret = perf_trace_event_init(tp_event, p_event);
-			break;
+		    tp_event->class && tp_event->class->reg) {
+		    	if (try_module_get(tp_event->mod)) {
+				ret = perf_trace_event_init(tp_event, p_event);
+				break;
+			}
 		}
 	}
 	mutex_unlock(&event_mutex);
@@ -138,6 +139,7 @@ void perf_trace_destroy(struct perf_event *p_event)
 
 	free_percpu(tp_event->perf_events);
 	tp_event->perf_events = NULL;
+	module_put(tp_event->mod);
 
 	if (!--total_ref_count) {
 		for (i = 0; i < 4; i++) {

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