linux-kernel - Re: [PATCH v2] tracing/events: don't discard an event after commit

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Date:	Mon, 23 Mar 2009 03:33:03 -0500
From:	Tom Zanussi <tzanussi@...il.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Steven Rostedt <rostedt@...dmis.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] tracing/events: don't discard an event after commit

Hi Frederic,

On Mon, 2009-03-23 at 00:18 +0100, Frederic Weisbecker wrote:
> Note: depends on "tracing/events: don't use wake up for events"
> 
> When we want to filter an event, the filter test is done after
> the event is commited to the ring-buffer to be discarded later if
> needed.
> 
> But a reader could be reading this event while we are trying to discard
> it. Other kind of racy events can even happen because the event is
> commited and can be read and/or consumed.
> 
> What we want is to discard the event before committing it.
> 

Thanks for fixing this - I had intend on doing this, but it completely
slipped my mind.

Thanks for all your other comments and fixes too...

Tom

> v2: add a reported-by Steven Rostedt
> 
> Reported-by: Steven Rostedt <rostedt@...dmis.org>
> Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
> ---
>  kernel/trace/trace_events_stage_3.h |    5 +++--
>  1 files changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/trace/trace_events_stage_3.h b/kernel/trace/trace_events_stage_3.h
> index 9a3bd49..9d2fa78 100644
> --- a/kernel/trace/trace_events_stage_3.h
> +++ b/kernel/trace/trace_events_stage_3.h
> @@ -222,10 +222,11 @@ static void ftrace_raw_event_##call(proto)				\
>  									\
>  	assign;								\
>  									\
> -	trace_nowake_buffer_unlock_commit(event, irq_flags, pc);	\
> -									\
>  	if (call->preds && !filter_match_preds(call, entry))		\
>  		ring_buffer_event_discard(event);			\
> +									\
> +	trace_nowake_buffer_unlock_commit(event, irq_flags, pc);	\
> +									\
>  }									\
>  									\
>  static int ftrace_raw_reg_event_##call(void)				\

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