From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qc0-f205.google.com (mail-qc0-f205.google.com [209.85.216.205]) by kanga.kvack.org (Postfix) with ESMTP id 011126B0037 for ; Wed, 30 Oct 2013 12:44:36 -0400 (EDT) Received: by mail-qc0-f205.google.com with SMTP id c3so22929qcv.0 for ; Wed, 30 Oct 2013 09:44:36 -0700 (PDT) Received: from psmtp.com ([74.125.245.103]) by mx.google.com with SMTP id kg8si16018698pad.241.2013.10.29.07.29.48 for ; Tue, 29 Oct 2013 07:29:48 -0700 (PDT) Date: Tue, 29 Oct 2013 10:26:44 -0400 From: Johannes Weiner Subject: Re: [PATCH v2 2/3] percpu: fix this_cpu_sub() subtrahend casting for unsigneds Message-ID: <20131029142644.GB1548@cmpxchg.org> References: <1382895017-19067-1-git-send-email-gthelen@google.com> <1382895017-19067-3-git-send-email-gthelen@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1382895017-19067-3-git-send-email-gthelen@google.com> Sender: owner-linux-mm@kvack.org List-ID: To: Greg Thelen Cc: Tejun Heo , Christoph Lameter , Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , Michal Hocko , Balbir Singh , KAMEZAWA Hiroyuki , handai.szj@taobao.com, Andrew Morton , x86@kernel.org, linux-kernel@vger.kernel.org, cgroups@vger.kernel.org, linux-mm@kvack.org On Sun, Oct 27, 2013 at 10:30:16AM -0700, Greg Thelen wrote: > this_cpu_sub() is implemented as negation and addition. > > This patch casts the adjustment to the counter type before negation to > sign extend the adjustment. This helps in cases where the counter > type is wider than an unsigned adjustment. An alternative to this > patch is to declare such operations unsupported, but it seemed useful > to avoid surprises. > > This patch specifically helps the following example: > unsigned int delta = 1 > preempt_disable() > this_cpu_write(long_counter, 0) > this_cpu_sub(long_counter, delta) > preempt_enable() > > Before this change long_counter on a 64 bit machine ends with value > 0xffffffff, rather than 0xffffffffffffffff. This is because > this_cpu_sub(pcp, delta) boils down to this_cpu_add(pcp, -delta), > which is basically: > long_counter = 0 + 0xffffffff > > Also apply the same cast to: > __this_cpu_sub() > __this_cpu_sub_return() > this_cpu_sub_return() > > All percpu_test.ko passes, especially the following cases which > previously failed: > > l -= ui_one; > __this_cpu_sub(long_counter, ui_one); > CHECK(l, long_counter, -1); > > l -= ui_one; > this_cpu_sub(long_counter, ui_one); > CHECK(l, long_counter, -1); > CHECK(l, long_counter, 0xffffffffffffffff); > > ul -= ui_one; > __this_cpu_sub(ulong_counter, ui_one); > CHECK(ul, ulong_counter, -1); > CHECK(ul, ulong_counter, 0xffffffffffffffff); > > ul = this_cpu_sub_return(ulong_counter, ui_one); > CHECK(ul, ulong_counter, 2); > > ul = __this_cpu_sub_return(ulong_counter, ui_one); > CHECK(ul, ulong_counter, 1); > > Signed-off-by: Greg Thelen > Acked-by: Tejun Heo FWIW: Acked-by: Johannes Weiner -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org