On Wed, 20 May 2026 22:08:01 -0400 Steven Rostedt <[email protected]> wrote:
> From c3651ad3ac95b331c7aa010d163704a3702855da Mon Sep 17 00:00:00 2001 > From: Steven Rostedt <[email protected]> > Date: Wed, 20 May 2026 14:28:17 -0400 > Subject: [PATCH] ring-buffer: Fix reporting of missed events in iterator > > 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. > Looks good to me. Acked-by: Masami Hiramatsu (Google) <[email protected]> > Cc: [email protected] > Fixes: c9b7a4a72ff64 ("ring-buffer/tracing: Have iterator acknowledge dropped > events") > Signed-off-by: Steven Rostedt <[email protected]> > --- > Changes since v1: > https://patch.msgid.link/[email protected] > > - Added clearing iter->missed_events in rb_iter_reset() (Sashiko) > > kernel/trace/ring_buffer.c | 8 +++----- > 1 file changed, 3 insertions(+), 5 deletions(-) > > diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c > index 7288383b1f27..7b07d2004cc6 100644 > --- a/kernel/trace/ring_buffer.c > +++ b/kernel/trace/ring_buffer.c > @@ -5429,6 +5429,7 @@ static void rb_iter_reset(struct ring_buffer_iter *iter) > iter->head_page = cpu_buffer->reader_page; > iter->head = cpu_buffer->reader_page->read; > iter->next_event = iter->head; > + iter->missed_events = 0; > > iter->cache_reader_page = iter->head_page; > iter->cache_read = cpu_buffer->read; > @@ -6108,10 +6109,7 @@ 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); > > @@ -6273,7 +6271,7 @@ 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) <[email protected]>
