Received: by 2002:a25:ab43:0:0:0:0:0 with SMTP id u61csp6531492ybi; Wed, 29 May 2019 09:08:45 -0700 (PDT) X-Google-Smtp-Source: APXvYqyX7Iq3Z7ic3KGwGEmB0t//UPe+pg5huIeiyrc1zj/wUoiWOv+EaqUkXPvA2YnCASGTmM/e X-Received: by 2002:a17:90a:bb82:: with SMTP id v2mr13623678pjr.73.1559146125130; Wed, 29 May 2019 09:08:45 -0700 (PDT) ARC-Seal: i=1; a=rsa-sha256; t=1559146125; cv=none; d=google.com; s=arc-20160816; b=sEafJFufLCw5S5vqTIMV/ErzY//nE/qY3BpWUv/tnTjrw+48SKBvidO2GQMFcCMXja 1BS2mbOrdqS53xjJ8jXavYLqxoH5gv40nzgLkNHqwm9uklq2NjivIz8LYJryt4kieesK c7rGQ/h9IAciMF+uOph9hmeuel52XUocpU97yPAxBVzIwnbZfxIJJoCir44l10vLHgk4 B+8mtFvlAjKidR+MiFYqMuQdiamQkc0CbhIQtEhGBEN/hEyXYbGRSgRH+OD9qB+mZp/a a7OkaQslqKM49H1GL51Mva+7v2BQNhNMhBdBdqPMMUkE9DNfqvkc5ArNrtIy0VEfhm9V nx3g== ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=arc-20160816; h=list-id:precedence:sender:message-id:date:content-transfer-encoding :content-id:mime-version:subject:cc:to:references:in-reply-to:from :organization; bh=ryUumiPtswPdt4a6EC+jsEBoXkXmpyML191TMWRWbqA=; b=ZbiQx1O+b9UrKADHZ4rXFMY3qippKffgijSgeuVj8VH2CIdu8Ezmyk/P+62UBKP3dL 5UR5SANRbV4uJI8UD3v3axlidMkMjPoUdL4ZRZ4tfxIAk9oegrLv1JRv9+U2+8bGPGmy TNZOJAIsIOs/M7i/LojmizoUNxdU6TcAxj+2CNli/b5oFgs4izFK/yeYYOxZDYUwpS+l +MK3GFWIyXWzFC/Uc507G2YEhcWAY4/lBs/w8oyIn/ENtdkWjXkzFDrJcZ1HJW1L6ocw t0oUDCSVE8hXLKvsC/y2L8R+bc3JdhFv8Wm0MiPLKjQmTUt7bailov5leN63JKD+PtsQ Cyhg== ARC-Authentication-Results: i=1; mx.google.com; spf=pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org; dmarc=fail (p=NONE sp=NONE dis=NONE) header.from=redhat.com Return-Path: Received: from vger.kernel.org (vger.kernel.org. [209.132.180.67]) by mx.google.com with ESMTP id n11si27773062plg.248.2019.05.29.09.08.27; Wed, 29 May 2019 09:08:45 -0700 (PDT) Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67; Authentication-Results: mx.google.com; spf=pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org; dmarc=fail (p=NONE sp=NONE dis=NONE) header.from=redhat.com Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727130AbfE2QGu convert rfc822-to-8bit (ORCPT + 99 others); Wed, 29 May 2019 12:06:50 -0400 Received: from mx1.redhat.com ([209.132.183.28]:54926 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726118AbfE2QGt (ORCPT ); Wed, 29 May 2019 12:06:49 -0400 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.phx2.redhat.com [10.5.11.15]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id F077C30BC572; Wed, 29 May 2019 16:06:44 +0000 (UTC) Received: from warthog.procyon.org.uk (ovpn-120-173.rdu2.redhat.com [10.10.120.173]) by smtp.corp.redhat.com (Postfix) with ESMTP id E295A7941C; Wed, 29 May 2019 16:06:40 +0000 (UTC) Organization: Red Hat UK Ltd. Registered Address: Red Hat UK Ltd, Amberley Place, 107-111 Peascod Street, Windsor, Berkshire, SI4 1TE, United Kingdom. Registered in England and Wales under Company Registration No. 3798903 From: David Howells In-Reply-To: <20190528231218.GA28384@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> To: Greg KH Cc: dhowells@redhat.com, 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 Subject: Re: [PATCH 1/7] General notification queue with user mmap()'able ring buffer MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-ID: <31935.1559146000.1@warthog.procyon.org.uk> Content-Transfer-Encoding: 8BIT Date: Wed, 29 May 2019 17:06:40 +0100 Message-ID: <31936.1559146000@warthog.procyon.org.uk> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.49]); Wed, 29 May 2019 16:06:48 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Greg KH wrote: > > kref_put() could potentially add an unnecessary extra stack frame and would > > seem to be best avoided, though an optimising compiler ought to be able to > > inline if it can. > > If kref_put() is on your fast path, you have worse problems (kfree isn't > fast, right?) > > Anyway, it's an inline function, how can it add an extra stack frame? The call to the function pointer. Hopefully the compiler will optimise that away for an inlineable function. > > Are you now on the convert all refcounts to krefs path? > > "now"? Remember, I wrote kref all those years ago, Yes - and I thought it wasn't a good idea at the time. But this is the first time you've mentioned it to me, let alone pushed to change to it, that I recall. > everyone should use > it. It saves us having to audit the same pattern over and over again. > And, even nicer, it uses a refcount now, and as you are trying to > reference count an object, it is exactly what this was written for. > > So yes, I do think it should be used here, unless it is deemed to not > fit the pattern/usage model. kref_put() enforces a very specific destructor signature. I know of places where that doesn't work because the destructor takes more than one argument (granted that this is not the case here). So why does kref_put() exist at all? Why not kref_dec_and_test()? Why doesn't refcount_t get merged into kref, or vice versa? Having both would seem redundant. Mind you, I've been gradually reverting atomic_t-to-refcount_t conversions because it seems I'm not allowed refcount_inc/dec_return() and I want to get at the point refcount for tracing purposes. > > > > +module_exit(watch_queue_exit); > > > > > > module_misc_device()? > > > > warthog>git grep module_misc_device -- Documentation/ > > warthog1> > > Do I have to document all helper macros? If you add an API, documenting it is your privilege ;-) It's an important test of the API - if you can't describe it, it's probably wrong. Now I will grant that you didn't add that function... > Anyway, it saves you boilerplate code, but if built in, it's at the module > init level, not the fs init level, like you are asking for here. So that > might not work, it's your call. Actually, I probably shouldn't have a module exit function. It can't be a module as it's called by core code. I'll switch to builtin_misc_device(). > 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 David