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 <eran...@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.
------------------------------------------------------------------------------
_______________________________________________
perfmon2-devel mailing list
perfmon2-devel@lists.sourceforge.net
https://lists.sourceforge.net/lists/listinfo/perfmon2-devel
