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

[John Kacur]

On Mon, 17 May 2010, 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. 

It does? Your code looks good to me, but the description above is
a little flawed, since no new label is added.

Thanks

> 
>   Signed-off-by: Stephane Eranian <eranian@xxxxxxxxxx>
> 
> --
> 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;
> +	}
>  
>  	have_lost = atomic_read(&data->lost);
>  	if (have_lost)
> --
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