Re: [4.9-rc1+] intel_uncore builtin + CONFIG_DEBUG_TEST_DRIVER_REMOVE kernel panic

[Jiri Olsa <jolsa@redhat.com>]

On Thu, Oct 20, 2016 at 07:39:44AM +0200, Peter Zijlstra wrote:
> On Wed, Oct 19, 2016 at 09:19:43PM +0200, Jiri Olsa wrote:
> > I think the reason here is that presume pmu devices are always added,
> > but we add them only if pmu_bus_running (in perf_event_sysfs_init)
> > is set which might happen after uncore initcall
> > 
> > attached patch fixes the issue for me
> 
> Right, we never expected to be unloaded before userspace runs.
> 
> Strictly speaking we should only read pmu_bus_running while holding
> pmus_lock, that way we're serialized against perf_event_sysfs_init()
> flipping it while we're being removed etc..
> 
> With the current setup the introduced race is harmless, but who knows
> what other crazy these device people will come up with ;-)
> 

right, did not think of that ;-)

also I did not noticed device_remove_file call for pmu->nr_addr_filters
and we could save one lock/unlock call later.. I'm testing attached patch
now

thanks,
jirka


---
diff --git a/kernel/events/core.c b/kernel/events/core.c
index c6e47e97b33f..224dffbc3b9b 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -8581,24 +8581,24 @@ static void update_pmu_context(struct pmu *pmu, struct pmu *old_pmu)
 	}
 }
 
+/*
+ * The pmus_lock lock must be taken.
+ */
 static void free_pmu_context(struct pmu *pmu)
 {
 	struct pmu *i;
 
-	mutex_lock(&pmus_lock);
 	/*
 	 * Like a real lame refcount.
 	 */
 	list_for_each_entry(i, &pmus, entry) {
 		if (i->pmu_cpu_context == pmu->pmu_cpu_context) {
 			update_pmu_context(i, pmu);
-			goto out;
+			return;
 		}
 	}
 
 	free_percpu(pmu->pmu_cpu_context);
-out:
-	mutex_unlock(&pmus_lock);
 }
 
 /*
@@ -8869,11 +8869,15 @@ void perf_pmu_unregister(struct pmu *pmu)
 	free_percpu(pmu->pmu_disable_count);
 	if (pmu->type >= PERF_TYPE_MAX)
 		idr_remove(&pmu_idr, pmu->type);
-	if (pmu->nr_addr_filters)
-		device_remove_file(pmu->dev, &dev_attr_nr_addr_filters);
-	device_del(pmu->dev);
-	put_device(pmu->dev);
+	mutex_lock(&pmus_lock);
+	if (pmu_bus_running) {
+		if (pmu->nr_addr_filters)
+			device_remove_file(pmu->dev, &dev_attr_nr_addr_filters);
+		device_del(pmu->dev);
+		put_device(pmu->dev);
+	}
 	free_pmu_context(pmu);
+	mutex_unlock(&pmus_lock);
 }
 EXPORT_SYMBOL_GPL(perf_pmu_unregister);