linux-kernel - Re: [PATCH v2 1/2] ring-buffer: Introducing ring-buffer mapping functions

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Message-ID: <20230330112103.00c3f422@gandalf.local.home>
Date:   Thu, 30 Mar 2023 11:21:03 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Vincent Donnefort <vdonnefort@...gle.com>
Cc:     mhiramat@...nel.org, linux-kernel@...r.kernel.org,
        linux-trace-kernel@...r.kernel.org, kernel-team@...roid.com
Subject: Re: [PATCH v2 1/2] ring-buffer: Introducing ring-buffer mapping
 functions

On Thu, 30 Mar 2023 11:30:51 +0100
Vincent Donnefort <vdonnefort@...gle.com> wrote:

> How about?
> 
> userspace:
> 
>   prev_read = meta->read;
>   ioctl(TRACE_MMAP_IOCTL_GET_READER_PAGE)
> 
> kernel:
>     ring_buffer_get_reader_page()
>       rb_get_reader_page(cpu_buffer);
>       cpu_buffer->reader_page->read = rb_page_size(reader);
>       meta->read = cpu_buffer->reader_page->read;
> 
> userspace:
>    /* if new page prev_read = 0 */
>    /* read between prev_read and meta->read */
> 
> If the writer does anything in-between, wouldn't rb_get_reader_page() handle it
> nicely by returning the same reader as more would be there to read?
> 
> It is similar to rb_advance_reader() except we'd be moving several events at
> once?

Yeah, I think that can work. So we just need to make sure that the meta
page has the "read" variable passed through.

-- Steve