Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757927Ab3J2O3r (ORCPT ); Tue, 29 Oct 2013 10:29:47 -0400 Received: from zene.cmpxchg.org ([85.214.230.12]:33313 "EHLO zene.cmpxchg.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756968Ab3J2O3p (ORCPT ); Tue, 29 Oct 2013 10:29:45 -0400 Date: Tue, 29 Oct 2013 10:26:44 -0400 From: Johannes Weiner 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 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: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2041 Lines: 61 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 from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/