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

[Haifeng Xu]

On 2024/4/18 00:43, Frederic Weisbecker wrote:
> 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?

Yes. I'll send next verion with your suggestions and add:

Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>

> 
> 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
>>
>>