Re: [PATCH 1/7] General notification queue with user mmap()'able ring buffer

From: Greg KH
Date: Wed May 29 2019 - 19:14:44 EST

On Wed, May 29, 2019 at 05:06:40PM +0100, David Howells wrote:
> Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
> > And how does the tracing and perf ring buffers do this without needing
> > volatile? Why not use the same type of interface they provide, as it's
> > always good to share code that has already had all of the nasty corner
> > cases worked out.
> I've no idea how trace does it - or even where - or even if. As far as I can
> see, grepping for mmap in kernel/trace/*, there's no mmap support.
> Reading Documentation/trace/ring-buffer-design.txt the trace subsystem has
> some sort of transient page fifo which is a lot more complicated than what I
> want and doesn't look like it'll be mmap'able.
> Looking at the perf ring buffer, there appears to be a missing barrier in
> perf_aux_output_end():
> rb->user_page->aux_head = rb->aux_head;
> should be:
> smp_store_release(&rb->user_page->aux_head, rb->aux_head);
> It should also be using smp_load_acquire(). See
> Documentation/core-api/circular-buffers.rst
> And a (partial) patch has been proposed:

So, if that's all that needs to be fixed, can you use the same
buffer/code if that patch is merged?


greg k-h