Re: [PATCH v3 3/7] perf: Simplify perf_event_free_task() wait

[Peter Zijlstra]

On Mon, Mar 17, 2025 at 12:19:07PM +0530, Ravi Bangoria wrote:
> Hi Peter,
> 
> > @@ -1223,8 +1223,14 @@ static void put_ctx(struct perf_event_co
> >  	if (refcount_dec_and_test(&ctx->refcount)) {
> >  		if (ctx->parent_ctx)
> >  			put_ctx(ctx->parent_ctx);
> > -		if (ctx->task && ctx->task != TASK_TOMBSTONE)
> > -			put_task_struct(ctx->task);
> > +		if (ctx->task) {
> > +			if (ctx->task == TASK_TOMBSTONE) {
> > +				smp_mb(); /* pairs with wait_var_event() */
> > +				wake_up_var(&ctx->refcount);
> 
> perf_event_free_task() waits on "ctx->refcount == 1". But moving
> wake_up_var() under refcount_dec_and_test() will cause
> perf_event_free_task() to wait indefinitely. Right? So, shouldn't
> wake_up_var() be outside? something like:
> 
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -1281,15 +1281,14 @@ static void put_ctx(struct perf_event_context *ctx)
>  	if (refcount_dec_and_test(&ctx->refcount)) {
>  		if (ctx->parent_ctx)
>  			put_ctx(ctx->parent_ctx);
> -		if (ctx->task) {
> -			if (ctx->task == TASK_TOMBSTONE) {
> -				smp_mb(); /* pairs with wait_var_event() */
> -				wake_up_var(&ctx->refcount);
> -			} else {
> -				put_task_struct(ctx->task);
> -			}
> -		}
> +		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);
> +		}
>  	}
>  }

Yes, you're quite right indeed. Thanks!