perf: Simplify perf_event_free_task() wait

Simplify the code by moving the duplicated wakeup condition into
put_ctx().

Notably, wait_var_event() is in perf_event_free_task() and will have
set ctx->task = TASK_TOMBSTONE.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Ravi Bangoria <ravi.bangoria@amd.com>
Link: https://lkml.kernel.org/r/20250307193723.044499344@infradead.org

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 3c92b75..fa6dab0 100644 (file)
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -1270,6 +1270,9 @@ static void put_ctx(struct perf_event_context *ctx)
                if (ctx->task && ctx->task != TASK_TOMBSTONE)
                        put_task_struct(ctx->task);
                call_rcu(&ctx->rcu_head, free_ctx);
+       } else if (ctx->task == TASK_TOMBSTONE) {
+               smp_mb(); /* pairs with wait_var_event() */
+               wake_up_var(&ctx->refcount);
        }
 }
 
@@ -5729,8 +5732,6 @@ int perf_event_release_kernel(struct perf_event *event)
 again:
        mutex_lock(&event->child_mutex);
        list_for_each_entry(child, &event->child_list, child_list) {
-               void *var = NULL;
-
                /*
                 * Cannot change, child events are not migrated, see the
                 * comment with perf_event_ctx_lock_nested().
@@ -5765,40 +5766,20 @@ again:
                if (tmp == child) {
                        perf_remove_from_context(child, DETACH_GROUP | DETACH_CHILD);
                        list_add(&child->child_list, &free_list);
-               } else {
-                       var = &ctx->refcount;
                }
 
                mutex_unlock(&event->child_mutex);
                mutex_unlock(&ctx->mutex);
                put_ctx(ctx);
 
-               if (var) {
-                       /*
-                        * If perf_event_free_task() has deleted all events from the
-                        * ctx while the child_mutex got released above, make sure to
-                        * notify about the preceding put_ctx().
-                        */
-                       smp_mb(); /* pairs with wait_var_event() */
-                       wake_up_var(var);
-               }
                goto again;
        }
        mutex_unlock(&event->child_mutex);
 
        list_for_each_entry_safe(child, tmp, &free_list, child_list) {
-               void *var = &child->ctx->refcount;
-
                list_del(&child->child_list);
                /* Last reference unless ->pending_task work is pending */
                put_event(child);
-
-               /*
-                * Wake any perf_event_free_task() waiting for this event to be
-                * freed.
-                */
-               smp_mb(); /* pairs with wait_var_event() */
-               wake_up_var(var);
        }
 
 no_ctx: