linux-kernel - Re: [PATCH v3 1/2] tracing: ring_buffer: Rewind persistent ring buffer when reboot

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Message-ID: <20250527094757.791e8f10@gandalf.local.home>
Date: Tue, 27 May 2025 09:47:57 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: "Masami Hiramatsu (Google)" <mhiramat@...nel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
 linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/2] tracing: ring_buffer: Rewind persistent ring
 buffer when reboot

On Tue, 27 May 2025 12:54:44 +0900
Masami Hiramatsu (Google) <mhiramat@...nel.org> wrote:

> Here is what I meant. As far as I ran my test, it looks good (it prevents
> over-read by `cat per_cpu/cpu0/trace_pipe_raw`)
> 
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index 5034bae02f08..de1831eb3446 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -5405,6 +5405,7 @@ rb_get_reader_page(struct ring_buffer_per_cpu *cpu_buffer)
>         unsigned long flags;
>         int nr_loops = 0;
>         bool ret;
> +       u64 ts;
>  
>         local_irq_save(flags);
>         arch_spin_lock(&cpu_buffer->lock);
> @@ -5423,6 +5424,18 @@ rb_get_reader_page(struct ring_buffer_per_cpu *cpu_buffer)
>  
>         reader = cpu_buffer->reader_page;
>  
> +       /*
> +        * Now the page->commit is not cleared when it read.
> +        * Check whether timestamp is newer instead. We also don't
> +        * care the head_page is overwritten. In that case, timestamp
> +        * should be newer than reader timestamp too.
> +        */
> +       ts = cpu_buffer->head_page->page->time_stamp;
> +       if (ts < reader->page->time_stamp) {

Hmm, I think this test may be too fragile. The head_page can be moved
by the writer, and this would need to handle races.

I found an issue with commit overflow and have a couple of bugs to fix that
touches some of this code. Let's revisit after I get those fixed.

Thanks,

-- Steve



> +               reader = NULL;
> +               goto out;
> +       }
> +
>         /* If there's more to read, return this page */
>         if (cpu_buffer->reader_page->read < rb_page_size(reader))
>                 goto out;
>