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

[Vincent Donnefort <vdonnefort@google.com>]

On Wed, Mar 29, 2023 at 09:36:46AM -0400, Steven Rostedt wrote:
> On Wed, 29 Mar 2023 14:31:07 +0100
> Vincent Donnefort <vdonnefort@google.com> wrote:
> 
> > We can say we update cpu_buffer->reader_page->read on the get_reader_page ioctl,
> > to the most recent value possible, which will have the consequence of actually
> > "flushing" those events?
> 
> Yes. It should be no different than doing a normal read of the
> trace_pipe_raw file, which does the same.
> 
> > 
> > If the reader decides to read events past this value then it just can't expect
> > them to not be duplicated?
> > 
> > I suppose it'd be down the reader to store meta->read somehwere?
> > 
> >    prev_read = meta->read
> >    ioctl(fd, TRACE_MMAP_IOCTL_GET_READER_PAGE)
> >    /* read events from prev_read to meta->read */
> 
> Yes, in fact it shouldn't need to call the ioctl until after it read it.
> 
> Maybe, we should have the ioctl take a parameter of how much was read?
> To prevent races?

Races would only be with other consuming readers. In that case we'd probably
have many other problems anyway as I suppose nothing would prevent another one
of swapping the page while our userspace reader is still processing it?

I don't know if this is worth splitting the ABI between the meta-page and the
ioctl parameters for this?

Or maybe we should say the meta-page contains things modified by the writer and
parameters modified by the reader are passed by the get_reader_page ioctl i.e.
the reader page ID and cpu_buffer->reader_page->read? (for the hyp tracing, we
have up to 4 registers for the HVC which would replace in our case the ioctl)

> 
> That is, it should pass in the page->commit that it used to to read the
> pages.
> 
> -- Steve