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.

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

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>

On Thu, 21 May 2026 10:29:21 +0900
Masami Hiramatsu (Google) <mhiramat@kernel.org> wrote:


> > 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?)
> 

Hmm, I think that was an issue even before this patch, but Sashiko is
right. It should be fixed. I'll send a v2.

Thanks,

-- Steve