[RFC 1/3] perf: Add new late event free callback

[Tvrtko Ursulin]

From: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>

This allows drivers to implement a PMU with support for unbinding the
device, for example by making event->pmu reference counted on the driver
side and its lifetime matching the struct perf_event init/free.

Otherwise, if an open perf fd is kept past driver unbind, the perf code
can dereference the potentially freed struct pmu from the _free_event
steps which follow the existing destroy callback.

TODO/FIXME/QQQ:

A simpler version could be to simply move the ->destroy() callback to
later in _free_event(). However a comment there claims there are steps
which need to run after the existing destroy callbacks, hence I opted for
an initially cautious approach.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@xxxxxxxxx>
Cc: Aravind Iddamsetty <aravind.iddamsetty@xxxxxxxxxxxxxxx>
---
 include/linux/perf_event.h |  1 +
 kernel/events/core.c       | 13 +++++++++++--
 2 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index 5547ba68e6e4..a567d2d98be1 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -799,6 +799,7 @@ struct perf_event {
 	struct perf_event		*aux_event;
 
 	void (*destroy)(struct perf_event *);
+	void (*free)(struct perf_event *);
 	struct rcu_head			rcu_head;
 
 	struct pid_namespace		*ns;
diff --git a/kernel/events/core.c b/kernel/events/core.c
index a64165af45c1..4b62d2201ca7 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -5242,6 +5242,9 @@ static void _free_event(struct perf_event *event)
 	exclusive_event_destroy(event);
 	module_put(event->pmu->module);
 
+	if (event->free)
+		event->free(event);
+
 	call_rcu(&event->rcu_head, free_event_rcu);
 }
 
@@ -11662,8 +11665,12 @@ static int perf_try_init_event(struct pmu *pmu, struct perf_event *event)
 		    event_has_any_exclude_flag(event))
 			ret = -EINVAL;
 
-		if (ret && event->destroy)
-			event->destroy(event);
+		if (ret) {
+			if (event->destroy)
+				event->destroy(event);
+			if (event->free)
+				event->free(event);
+		}
 	}
 
 	if (ret)
@@ -12090,6 +12097,8 @@ perf_event_alloc(struct perf_event_attr *attr, int cpu,
 		perf_detach_cgroup(event);
 	if (event->destroy)
 		event->destroy(event);
+	if (event->free)
+		event->free(event);
 	module_put(pmu->module);
 err_ns:
 	if (event->hw.target)
-- 
2.40.1