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

[Greg KH]

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: https://lkml.org/lkml/2018/5/10/249

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

thanks,

greg k-h