Re: [PATCH] perf_events: fix errors path in perf_output_begin()

[Frederic Weisbecker <fweisbec@gmail.com>]

On Mon, May 17, 2010 at 12:46:01PM +0200, Stephane Eranian wrote:
>   In case the sampling buffer has no "payload" pages, nr_pages is 0.
>   The problem is that the error path in perf_output_begin() skips to
>   a label which assumes perf_output_lock() has been issued which is
>   not the case. That triggers a WARN_ON() is perf_output_unlock().
> 
>   This patch fixes the problem by adding a new label and skipping
>   perf_task_unlock() in case data->nr_pages is 0. 
> 
>   Signed-off-by: Stephane Eranian <eranian@google.com>
> 
> --
> diff --git a/kernel/perf_event.c b/kernel/perf_event.c
> index a4fa381..95137b6 100644
> --- a/kernel/perf_event.c
> +++ b/kernel/perf_event.c
> @@ -3035,8 +3035,10 @@ int perf_output_begin(struct perf_output_handle *handle,
>  	handle->nmi	= nmi;
>  	handle->sample	= sample;
>  
> -	if (!data->nr_pages)
> -		goto fail;
> +	if (!data->nr_pages) {
> +		atomic_inc(&data->lost);
> +		goto out;
> +	}



Oh indeed, handle->lock is in a random state.
Whatever its value we have an unbalanced put_cpu()
anyway.

And we don't race with someone else, data->lock = -1
and will then warn.

I just have a tiny doubt: should we really count this
path to the lost events? I'm not sure when we can have
data->nr_pages == 0, does this happen if we mmap after
enabling the event?

All I know is that I observed I already lost events in
this path using perf lock.