perf: Robustify event->owner usage and SMP ordering

Use smp_store_release() to clear event->owner and
lockless_dereference() to observe it. Further use READ_ONCE() for all
lockless reads.

This changes perf_remove_from_owner() to leave event->owner cleared.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vince Weaver <vincent.weaver@maine.edu>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 10 insertions, 10 deletions

diff --git a/kernel/events/core.c b/kernel/events/core.c
index d84374fa44e5..5f055de90c6d 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -152,7 +152,7 @@ static void perf_ctx_unlock(struct perf_cpu_context *cpuctx,
 
 static bool is_kernel_event(struct perf_event *event)
 {
-        return event->owner == TASK_TOMBSTONE;
+        return READ_ONCE(event->owner) == TASK_TOMBSTONE;
 }
 
 /*
@@ -1651,7 +1651,7 @@ out:
  */
 static bool is_orphaned_event(struct perf_event *event)
 {
-        return event && !is_kernel_event(event) && !event->owner;
+        return event && !is_kernel_event(event) && !READ_ONCE(event->owner);
 }
 
 /*
@@ -3733,14 +3733,13 @@ static void perf_remove_from_owner(struct perf_event *event)
         struct task_struct *owner;
 
         rcu_read_lock();
-        owner = ACCESS_ONCE(event->owner);
         /*
-         * Matches the smp_wmb() in perf_event_exit_task(). If we observe
-         * !owner it means the list deletion is complete and we can indeed
-         * free this event, otherwise we need to serialize on
+         * Matches the smp_store_release() in perf_event_exit_task(). If we
+         * observe !owner it means the list deletion is complete and we can
+         * indeed free this event, otherwise we need to serialize on
          * owner->perf_event_mutex.
          */
-        smp_read_barrier_depends();
+        owner = lockless_dereference(event->owner);
         if (owner) {
                 /*
                  * Since delayed_put_task_struct() also drops the last
@@ -3768,8 +3767,10 @@ static void perf_remove_from_owner(struct perf_event *event)
                  * ensured they're done, and we can proceed with freeing the
                  * event.
                  */
-                if (event->owner)
+                if (event->owner) {
                         list_del_init(&event->owner_entry);
+                        smp_store_release(&event->owner, NULL);
+                }
                 mutex_unlock(&owner->perf_event_mutex);
                 put_task_struct(owner);
         }
@@ -8829,8 +8830,7 @@ void perf_event_exit_task(struct task_struct *child)
                  * the owner, closes a race against perf_release() where
                  * we need to serialize on the owner->perf_event_mutex.
                  */
-                smp_wmb();
-                event->owner = NULL;
+                smp_store_release(&event->owner, NULL);
         }
         mutex_unlock(&child->perf_event_mutex);