perf: Drop module reference on event init failure

When initialising an event, perf_init_event will call try_module_get() to
ensure that the PMU's module cannot be removed for the lifetime of the
event, with __free_event() dropping the reference when the event is
finally destroyed. If something fails after the event has been
initialised, but before the event is installed, perf_event_alloc will
drop the reference on the module.

However, if we fail to initialise an event for some reason (e.g. we ask
an uncore PMU to perform sampling, and it refuses to initialise the
event), we do not drop the refcount. If we try to open such a bogus
event without a precise IDR type, we will loop over each PMU in the pmus
list, incrementing each of their refcounts without decrementing them.

This patch adds a module_put when pmu->event_init(event) fails, ensuring
that the refcounts are balanced in failure cases. As the innards of the
precise and search based initialisation look very similar, this logic is
hoisted out into a new helper function. While the early return for the
failed try_module_get is removed from the search case, this is handled
by the remaining return when ret is not -ENOENT.
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/1420642611-22667-1-git-send-email-mark.rutland@arm.comSigned-off-by: Ingo Molnar <mingo@kernel.org>

kernel/events/core.c

@@ -7027,6 +7027,20 @@ void perf_pmu_unregister(struct pmu *pmu)
 }
 EXPORT_SYMBOL_GPL(perf_pmu_unregister);
 
+static int perf_try_init_event(struct pmu *pmu, struct perf_event *event)
+{
+	int ret;
+
+	if (!try_module_get(pmu->module))
+		return -ENODEV;
+	event->pmu = pmu;
+	ret = pmu->event_init(event);
+	if (ret)
+		module_put(pmu->module);
+
+	return ret;
+}
+
 struct pmu *perf_init_event(struct perf_event *event)
 {
 	struct pmu *pmu = NULL;
@@ -7039,24 +7053,14 @@ struct pmu *perf_init_event(struct perf_event *event)
 	pmu = idr_find(&pmu_idr, event->attr.type);
 	rcu_read_unlock();
 	if (pmu) {
-		if (!try_module_get(pmu->module)) {
-			pmu = ERR_PTR(-ENODEV);
-			goto unlock;
-		}
-		event->pmu = pmu;
-		ret = pmu->event_init(event);
+		ret = perf_try_init_event(pmu, event);
 		if (ret)
 			pmu = ERR_PTR(ret);
 		goto unlock;
 	}
 
 	list_for_each_entry_rcu(pmu, &pmus, entry) {
-		if (!try_module_get(pmu->module)) {
-			pmu = ERR_PTR(-ENODEV);
-			goto unlock;
-		}
-		event->pmu = pmu;
-		ret = pmu->event_init(event);
+		ret = perf_try_init_event(pmu, event);
 		if (!ret)
 			goto unlock;