From mboxrd@z Thu Jan 1 00:00:00 1970 From: Johannes Weiner Subject: Re: [PATCH v2 2/3] percpu: fix this_cpu_sub() subtrahend casting for unsigneds Date: Tue, 29 Oct 2013 10:26:44 -0400 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 Return-path: DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=cmpxchg.org; s=zene; h=In-Reply-To:Content-Type:MIME-Version:References:Message-ID:Subject:Cc:To:From:Date; bh=Jcl7wRr7M1JyuLZiOtu1SKauIJkRc36Jn1UbG/+2dCE=; b=qt7qf4F7Gj/yKm8DcuQZLQUQV/yJnHaQ18f2OCFEIrCccaYrfzx3arC/c7LRXyIcft/6cLwaslkNhqBu7Ld97P7TqyC+gkQSRDPH/g1QgDzHRCQPHyUzadZjA5AozdJZY49YTJcsIJ3zRbqtaVubn2KYMfaYc4/BQB4BqrZwCKo=; Content-Disposition: inline In-Reply-To: <1382895017-19067-3-git-send-email-gthelen-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org> Sender: cgroups-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-ID: Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Greg Thelen Cc: Tejun Heo , Christoph Lameter , Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , Michal Hocko , Balbir Singh , KAMEZAWA Hiroyuki , handai.szj-3b8fjiQLQpfQT0dZR+AlfA@public.gmane.org, Andrew Morton , x86-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.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