Re: [PATCH] ring-buffer: Fix reporting of missed events in iterator

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Wed, 20 May 2026 14:28:17 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> From: Steven Rostedt <rostedt@goodmis.org>
> 
> When tracing is active while reading the trace file, if the iterator
> reading the buffer detects that the writer has passed the iterator head,
> it will reset and set a "missed events" flag. This flag is passed to the
> output processing to show the user that events were missed:
> 
>   CPU:4 [LOST EVENTS]
> 
> The problem is that the flag is reset after it is checked in
> ring_buffer_iter_dropped(). But the "trace" file iterates over all the CPU
> ring buffers and it will check if they are dropped when figuring out which
> buffer to print next. This prematurely clears the missed_events flag if
> the CPU buffer with the missed events is not the one that is printed next.
> 
> On the iteration where the CPU buffer with the missed events is printed,
> the check if it had missed events would return false and the output does
> not show that events were missed.
> 
> Do not reset the missed_events flag when checking if there were missed
> events, but instead clear it when moving the iterator head to the next
> event.
> 

As Sashiko pointed, this flag should be reset in rb_iter_reset() too.
(But that seems like another issue?)

Thank you,

> Cc: stable@vger.kernel.org
> Fixes: c9b7a4a72ff64 ("ring-buffer/tracing: Have iterator acknowledge dropped events")
> Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
> ---
>  kernel/trace/ring_buffer.c | 7 ++-----
>  1 file changed, 2 insertions(+), 5 deletions(-)
> 
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index 5326924615a4..47b0a7b43f0f 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -6086,10 +6086,7 @@ ring_buffer_peek(struct trace_buffer *buffer, int cpu, u64 *ts,
>   */
>  bool ring_buffer_iter_dropped(struct ring_buffer_iter *iter)
>  {
> -	bool ret = iter->missed_events != 0;
> -
> -	iter->missed_events = 0;
> -	return ret;
> +	return iter->missed_events != 0;
>  }
>  EXPORT_SYMBOL_GPL(ring_buffer_iter_dropped);
>  
> @@ -6251,7 +6248,7 @@ void ring_buffer_iter_advance(struct ring_buffer_iter *iter)
>  	unsigned long flags;
>  
>  	raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
> -
> +	iter->missed_events = 0;
>  	rb_advance_iter(iter);
>  
>  	raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
> -- 
> 2.53.0
> 
> 


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>