From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wr0-f193.google.com ([209.85.128.193]:36979 "EHLO mail-wr0-f193.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751343AbdITUyi (ORCPT ); Wed, 20 Sep 2017 16:54:38 -0400 Date: Wed, 20 Sep 2017 23:54:33 +0300 From: Alexey Dobriyan To: Paul Burton Cc: Heiko Carstens , Martin Schwidefsky , linux-s390@vger.kernel.org, Christian Borntraeger , Andrew Morton , stable Subject: Re: [PATCH] s390: Fix perf event init Message-ID: <20170920205433.GA1955@avx2> References: <20170920050757.22857-1-paul.burton@imgtec.com> <20170920085100.6adb0e0d@mschwideX1> <20170920121517.GE3583@osiris> <2025443.x60lSU25Yf@np-p-burton> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <2025443.x60lSU25Yf@np-p-burton> Sender: stable-owner@vger.kernel.org List-ID: On Wed, Sep 20, 2017 at 09:04:38AM -0700, Paul Burton wrote: > Hi Heiko, > > On Wednesday, 20 September 2017 05:15:17 PDT Heiko Carstens wrote: > > On Wed, Sep 20, 2017 at 08:51:00AM +0200, Martin Schwidefsky wrote: > > > Hi Paul, > > > > > > On Tue, 19 Sep 2017 22:07:57 -0700 > > > > > > Paul Burton wrote: > > > > Commit c311c797998c ("cpumask: make "nr_cpumask_bits" unsigned") > > > > modified cpumsf_pmu_event_init() to cast the struct perf_event cpu field > > > > to an unsigned integer before it is compared with nr_cpumask_bits. This > > > > is broken because the cpu field may be -1 for events which follow a > > > > process rather than being affine to a particular CPU. When this is the > > > > case the cast to an unsigned int results in a value equal to ULONG_MAX, > > > > which is always greater than nr_cpumask_bits so we always fail > > > > cpumsf_pmu_event_init() and return -ENODEV. > > > > > > > > The check against nr_cpumask_bits seems nonsensical anyway, so this > > > > patch simply removes it. The cpu field is going to either be 0 or a > > > > I assume you meant to write "...either be -1..."? > > Indeed I did... and I wrote -1 in the MIPS patch which I then copied this > from, so I'm not sure how it became 0. Oops! > > > > > valid CPU number. Comparing it with nr_cpumask_bits is effectively > > > > checking that it's a valid cpu number, but it seems safe to rely on the > > > > core perf events code to ensure that's the case. > > > > Looks like you are right and the nr_cpumask_bits check is not needed. The > > sanity check is done at the beginning of perf_event_alloc() and everything > > else can rely on a sane cpu number (-1 or within bounds of nr_cpumask_bits). > > > Thanks for the patch, there is indeed an issue with nr_cpumask_bits. > > > But we already have a slightly different fix for this queued on the > > > fixes branch of s390/linux: > > > > > > https://git.kernel.org/pub/scm/linux/kernel/git/s390/linux.git/commit/?h=f > > > ixes&id=fc3100d64f0ae383ae8d845989103da06d62763b > > So we should either use your patch or remove the superfluous check with an > > addon patch. Martin's call ;) > > Glad the patch is of use either way. First, sorry for the breakage. Second, if you write code like this: if (event->cpu >= 0) { if ((unsigned int)event->cpu >= nr_cpumask_bits) return -ENODEV; then cast is unnecessary as comparison is unsigned and non-negative values will remain themselves.