[PATCH] perf/core: Fix missing wakeup when waiting for context reference

[Haifeng Xu <haifeng.xu@shopee.com>]

In our production environment, we found many hung tasks which are
blocked for more than 18 hours. Their call traces are like this:

[346278.191038] __schedule+0x2d8/0x890
[346278.191046] schedule+0x4e/0xb0
[346278.191049] perf_event_free_task+0x220/0x270
[346278.191056] ? init_wait_var_entry+0x50/0x50
[346278.191060] copy_process+0x663/0x18d0
[346278.191068] kernel_clone+0x9d/0x3d0
[346278.191072] __do_sys_clone+0x5d/0x80
[346278.191076] __x64_sys_clone+0x25/0x30
[346278.191079] do_syscall_64+0x5c/0xc0
[346278.191083] ? syscall_exit_to_user_mode+0x27/0x50
[346278.191086] ? do_syscall_64+0x69/0xc0
[346278.191088] ? irqentry_exit_to_user_mode+0x9/0x20
[346278.191092] ? irqentry_exit+0x19/0x30
[346278.191095] ? exc_page_fault+0x89/0x160
[346278.191097] ? asm_exc_page_fault+0x8/0x30
[346278.191102] entry_SYSCALL_64_after_hwframe+0x44/0xae

The task was waiting for the refcount become to 1, but from the vmcore,
we found the refcount has already been 1. It seems that the task didn't
get woken up by perf_event_release_kernel() and got stuck forever. The
below scenario may cause the problem.

Thread A					Thread B
...						...
perf_event_free_task				perf_event_release_kernel
						   ...
						   acquire event->child_mutex
						   ...
						   get_ctx
   ...						   release event->child_mutex
   acquire ctx->mutex
   ...
   perf_free_event (acquire/release event->child_mutex)
   ...
   release ctx->mutex
   wait_var_event
						   acquire ctx->mutex
						   acquire event->mutex
						   # move existing events to free_list
						   release event->child_mutex
						   release ctx->mutex
						   put_ctx
...						...

In this case, all events of the ctx have been freed, so we couldn't
find the ctx in free_list and Thread A will miss the wakeup. It's thus
necessary to add a wakeup after dropping the reference.

Signed-off-by: Haifeng Xu <haifeng.xu@shopee.com>
---
 kernel/events/core.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 4f0c45ab8d7d..01dfe715f09e 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -5340,6 +5340,8 @@ 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;
+		bool freed = false;
 
 		/*
 		 * Cannot change, child events are not migrated, see the
@@ -5380,11 +5382,25 @@ int perf_event_release_kernel(struct perf_event *event)
 			 * this can't be the last reference.
 			 */
 			put_event(event);
+		} else {
+			freed = true;
+			var = &ctx->refcount;
 		}
 
 		mutex_unlock(&event->child_mutex);
 		mutex_unlock(&ctx->mutex);
 		put_ctx(ctx);
+
+		if (freed) {
+			/*
+			 * perf_event_free_task() delete all events of the ctx and
+			 * there is no event of the ctx in free_list. It may step
+			 * into wait_var_event() before decrement the refcount. So
+			 * we should add a wakeup here.
+			 */
+			smp_mb(); /* pairs with wait_var_event() */
+			wake_up_var(var);
+		}
 		goto again;
 	}
 	mutex_unlock(&event->child_mutex);
-- 
2.25.1