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

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Message-ID: <20250523172857.02ab4a75@gandalf.local.home>
Date: Fri, 23 May 2025 17:28:57 -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 Fri, 23 May 2025 16:54:25 -0400
Steven Rostedt <rostedt@...dmis.org> wrote:

> I think we also need this:
> 
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index 7d837963dd1e..456efebc396a 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -3638,6 +3638,9 @@ rb_move_tail(struct ring_buffer_per_cpu *cpu_buffer,
>  
>  	rb_reset_tail(cpu_buffer, tail, info);
>  
> +	/* The new page should have zero committed */
> +	rb_init_page(next_page->page);
> +
>  	/* Commit what we have for now. */
>  	rb_end_commit(cpu_buffer);
>  	/* rb_end_commit() decs committing */

No we don't need it ;-)

I'm looking deeper into the code and we have this:

		/*
		 * No need to worry about races with clearing out the commit.
		 * it only can increment when a commit takes place. But that
		 * only happens in the outer most nested commit.
		 */
		local_set(&next_page->page->commit, 0);

When the tail page gets moved.

-- Steve