Re: [PATCH 2/8] perf: Sanitize get_callchain_buffer()

[Jiri Olsa]

On Tue, Jul 23, 2013 at 02:31:00AM +0200, Frederic Weisbecker wrote:
> In case of allocation failure, get_callchain_buffer() keeps the
> refcount incremented for the current event.
> 
> As a result, when get_callchain_buffers() returns an error,
> we must cleanup what it did by cancelling its last refcount
> with a call to put_callchain_buffers().
> 
> This is a hack in order to be able to call free_event()
> after that failure.
> 
> The original purpose of that was to simplify the failure
> path. But this error handling is actually counter intuitive,
> ugly and not very easy to follow because one expect to
> see the resources used to perform a service to be cleaned
> by the callee if case of failure, not by the caller.
> 
> So lets clean this up by cancelling the refcount from
> get_callchain_buffer() in case of failure. And correctly free
> the event accordingly in perf_event_alloc().
> 
> Signed-off-by: Frederic Weisbecker <fweisbec@xxxxxxxxx>
> Cc: Jiri Olsa <jolsa@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Namhyung Kim <namhyung@xxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
> Cc: Stephane Eranian <eranian@xxxxxxxxxx>
> ---
>  kernel/events/callchain.c |    2 ++
>  kernel/events/core.c      |   41 +++++++++++++++++++++--------------------
>  2 files changed, 23 insertions(+), 20 deletions(-)
> 
> diff --git a/kernel/events/callchain.c b/kernel/events/callchain.c
> index c772061..76a8bc5 100644
> --- a/kernel/events/callchain.c
> +++ b/kernel/events/callchain.c
> @@ -117,6 +117,8 @@ int get_callchain_buffers(void)
>  	err = alloc_callchain_buffers();
>  exit:
>  	mutex_unlock(&callchain_mutex);
> +	if (err)
> +		atomic_dec(&nr_callchain_events);

shouldn't we touch this under above lock?

also that above hunk decrements nr_callchain_events
also for following case:

        count = atomic_inc_return(&nr_callchain_events);
        if (WARN_ON_ONCE(count < 1)) {
                err = -EINVAL;
                goto exit;
        }

seems like it screws the count

jirka
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