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
