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

[Frederic Weisbecker]

Le Wed, Apr 10, 2024 at 03:55:06AM +0000, Haifeng Xu a écrit :
> 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@xxxxxxxxxx>
> ---
>  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;

Good catch!

How about the following slightly simplified version?

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 724e6d7e128f..4082d0161b2b 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -5365,6 +5365,7 @@ 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
@@ -5405,11 +5406,23 @@ int perf_event_release_kernel(struct perf_event *event)
 			 * this can't be the last reference.
 			 */
 			put_event(event);
+		} 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);



>  	}
>  	mutex_unlock(&event->child_mutex);
> -- 
> 2.25.1
> 
>