On Wed, May 29, 2019 at 05:06:40PM +0100, David Howells wrote:

Greg KH [off-list ref] wrote:

quoted

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