From mboxrd@z Thu Jan 1 00:00:00 1970 From: Greg KH Subject: Re: [PATCH 1/7] General notification queue with user mmap()'able ring buffer Date: Wed, 29 May 2019 16:11:12 -0700 Message-ID: <20190529231112.GB3164@kroah.com> References: <20190528231218.GA28384@kroah.com> <20190528162603.GA24097@kroah.com> <155905930702.7587.7100265859075976147.stgit@warthog.procyon.org.uk> <155905931502.7587.11705449537368497489.stgit@warthog.procyon.org.uk> <4031.1559064620@warthog.procyon.org.uk> <31936.1559146000@warthog.procyon.org.uk> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <31936.1559146000@warthog.procyon.org.uk> Sender: linux-kernel-owner@vger.kernel.org To: David Howells Cc: viro@zeniv.linux.org.uk, raven@themaw.net, linux-fsdevel@vger.kernel.org, linux-api@vger.kernel.org, linux-block@vger.kernel.org, keyrings@vger.kernel.org, linux-security-module@vger.kernel.org, linux-kernel@vger.kernel.org List-Id: linux-api@vger.kernel.org On Wed, May 29, 2019 at 05:06:40PM +0100, David Howells wrote: > Greg KH 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