Re: [BUG] perf/core: Task stuck on global_ctx_data_rwsem

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Jan 07, 2026 at 11:01:53AM -0800, Namhyung Kim wrote:

> > But yes, I suppose this can do. The question is however, how do you get
> > into this predicament to begin with? Are you creating and destroying a
> > lot of global LBR events or something?
> 
> I think it's just because there are too many tasks in the system like
> O(100K).  And any thread going to exit needs to wait for
> attach_global_ctx_data() to finish the iteration over every task.

OMG, so many tasks ...

> > Would it make sense to delay detach_global_ctx_data() for a second or
> > so? That is, what is your event creation pattern?
> 
> I don't think it has a special pattern, but I'm curious how we can
> handle a race like below.
> 
>   attach_global_ctx_data
>     check p->flags & PF_EXITING
>                                               do_exit
>     (preemption)                                set PF_EXITING
>                                                 detach_task_ctx_data()
>     check p->perf_ctx_data
>     attach_task_ctx_data()   ---> memory leak

Oh right. Something like so perhaps?

---
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 3c2a491200c6..e5e716420eb3 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -5421,9 +5421,19 @@ attach_task_ctx_data(struct task_struct *task, struct kmem_cache *ctx_cache,
 		return -ENOMEM;
 
 	for (;;) {
-		if (try_cmpxchg((struct perf_ctx_data **)&task->perf_ctx_data, &old, cd)) {
+		if (try_cmpxchg(&task->perf_ctx_data, &old, cd)) {
 			if (old)
 				perf_free_ctx_data_rcu(old);
+			/*
+			 * try_cmpxchg() pairs with try_cmpxchg() from
+			 * detach_task_ctx_data() such that
+			 * if we race with perf_event_exit_task(), we must
+			 * observe PF_EXITING.
+			 */
+			if (task->flags & PF_EXITING) {
+				task->perf_ctx_data = NULL;
+				perf_free_ctx_data_rcu(cd);
+			}
 			return 0;
 		}
 
@@ -5469,6 +5479,8 @@ attach_global_ctx_data(struct kmem_cache *ctx_cache)
 	/* Allocate everything */
 	scoped_guard (rcu) {
 		for_each_process_thread(g, p) {
+			if (p->flags & PF_EXITING)
+				continue;
 			cd = rcu_dereference(p->perf_ctx_data);
 			if (cd && !cd->global) {
 				cd->global = 1;
@@ -14568,8 +14580,11 @@ void perf_event_exit_task(struct task_struct *task)
 
 	/*
 	 * Detach the perf_ctx_data for the system-wide event.
+	 *
+	 * Done without holding global_ctx_data_rwsem; typically
+	 * attach_global_ctx_data() will skip over this task, but otherwise
+	 * attach_task_ctx_data() will observe PF_EXITING.
 	 */
-	guard(percpu_read)(&global_ctx_data_rwsem);
 	detach_task_ctx_data(task);
 }