Re: [PATCH 1/7] perf: propagate perf_install_in_context errors up

[Jiri Olsa <jolsa@redhat.com>]

On Mon, Jul 01, 2019 at 11:59:49PM -0700, Ian Rogers wrote:
> The current __perf_install_in_context can fail and the error is ignored.
> Changing __perf_install_in_context can add new failure modes that need
> errors propagating up. This change prepares for this.
> 
> Signed-off-by: Ian Rogers <irogers@google.com>
> ---
>  kernel/events/core.c | 38 +++++++++++++++++++++++++-------------
>  1 file changed, 25 insertions(+), 13 deletions(-)
> 
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 785d708f8553..4faa90f5a934 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -2558,11 +2558,12 @@ static int  __perf_install_in_context(void *info)
>   *
>   * Very similar to event_function_call, see comment there.
>   */
> -static void
> +static int
>  perf_install_in_context(struct perf_event_context *ctx,
>  			struct perf_event *event,
>  			int cpu)
>  {
> +	int err;
>  	struct task_struct *task = READ_ONCE(ctx->task);
>  
>  	lockdep_assert_held(&ctx->mutex);
> @@ -2577,15 +2578,15 @@ perf_install_in_context(struct perf_event_context *ctx,
>  	smp_store_release(&event->ctx, ctx);
>  
>  	if (!task) {
> -		cpu_function_call(cpu, __perf_install_in_context, event);
> -		return;
> +		err = cpu_function_call(cpu, __perf_install_in_context, event);
> +		return err;
>  	}
>  
>  	/*
>  	 * Should not happen, we validate the ctx is still alive before calling.
>  	 */
>  	if (WARN_ON_ONCE(task == TASK_TOMBSTONE))
> -		return;
> +		return 0;
>  
>  	/*
>  	 * Installing events is tricky because we cannot rely on ctx->is_active
> @@ -2619,8 +2620,9 @@ perf_install_in_context(struct perf_event_context *ctx,
>  	 */
>  	smp_mb();
>  again:
> -	if (!task_function_call(task, __perf_install_in_context, event))
> -		return;
> +	err = task_function_call(task, __perf_install_in_context, event);
> +	if (err)
> +		return err;

you need to return in here if task_function_call succeeds and
continue in case of error, not the other way round, otherwise
bad things will happen ;-)

jirka

>  
>  	raw_spin_lock_irq(&ctx->lock);
>  	task = ctx->task;
> @@ -2631,7 +2633,7 @@ perf_install_in_context(struct perf_event_context *ctx,
>  		 * against perf_event_exit_task_context().
>  		 */
>  		raw_spin_unlock_irq(&ctx->lock);
> -		return;
> +		return 0;
>  	}
>  	/*
>  	 * If the task is not running, ctx->lock will avoid it becoming so,
> @@ -2643,6 +2645,7 @@ perf_install_in_context(struct perf_event_context *ctx,
>  	}
>  	add_event_to_ctx(event, ctx);
>  	raw_spin_unlock_irq(&ctx->lock);
> +	return 0;
>  }
>  
>  /*

SNIP