Re: [PATCH] ring-buffer: Better comment the use of RB_MISSED_EVENTS

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

On Thu, 28 May 2026 22:37:38 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> From: Steven Rostedt <rostedt@goodmis.org>
> 
> If the persistent ring buffer is detected on boot up to have a corrupted
> sub-buffer, that sub-buffer is cleared to zero and its commit value has
> the RB_MISSED_EVENTS bit set. That bit is to allow the "trace",
> "trace_pipe" and "trace_pipe_raw" files know that events were dropped by
> outputting "[LOST EVENTS]".
> 
> Only in this case does that bit get set in the writeable portion of the
> ring buffer. When events are dropped in the normal ring buffer, that
> information is stored in the cpu_buffer descriptor and the
> RB_MISSED_EVENTS is set in the buffer page at the time the page is
> consumed. It is never set in the writeable portion of the buffer.
> 
> Add comments to describe this better as it can be confusing to know when
> the RB_MISSED_EVENTS are set in the commit portion of the buffer page.
> 
> Link: https://lore.kernel.org/all/20260529001500.14178455a046a5cbc6180861@kernel.org/
> 

This looks good to me.

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

Thanks, 

> Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
> ---
>  kernel/trace/ring_buffer.c | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
> 
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index 910f6b3adf74..06fb365bb86e 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -1929,6 +1929,12 @@ static int __rb_validate_buffer(struct buffer_page *bpage, int cpu,
>  	 */
>  	if (ret < 0 || (prev_ts && prev_ts > ts) || (next_ts && ts > next_ts)) {
>  		local_set(&bpage->entries, 0);
> +		/*
> +		 * Note, the RB_MISSED_EVENTS is only set inside the main write
> +		 * buffer by this verification logic. The normal ring buffer
> +		 * has this bit set when the page is read and passed to the
> +		 * consumers.
> +		 */
>  		local_set(&dpage->commit, RB_MISSED_EVENTS);
>  		dpage->time_stamp = prev_ts ? prev_ts : next_ts;
>  		ret = -1;
> @@ -7232,6 +7238,14 @@ int ring_buffer_read_page(struct trace_buffer *buffer,
>  			local_add(RB_MISSED_STORED, &dpage->commit);
>  			size += sizeof(missed_events);
>  		}
> +		/*
> +		 * Note, for the persistent ring buffer, the RB_MISSED_EVENTS
> +		 * may have been set in the main buffer via the verification code.
> +		 * But here, dpage is a copy of that page and has not yet had
> +		 * the RB_MISSED_EVENTS set. As for the normal buffers,
> +		 * the main write buffer does not set these bits and it needs
> +		 * to be set here.
> +		 */
>  		local_add(RB_MISSED_EVENTS, &dpage->commit);
>  	}
>  
> -- 
> 2.53.0
> 


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