From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pa0-f51.google.com (mail-pa0-f51.google.com [209.85.220.51]) (using TLSv1 with cipher ECDHE-RSA-RC4-SHA (128/128 bits)) (No client certificate requested) by ozlabs.org (Postfix) with ESMTPS id 0E0582C00C5 for ; Sat, 15 Mar 2014 06:10:59 +1100 (EST) Received: by mail-pa0-f51.google.com with SMTP id kq14so3036465pab.38 for ; Fri, 14 Mar 2014 12:10:57 -0700 (PDT) Date: Fri, 14 Mar 2014 12:10:55 -0700 From: Christoffer Dall To: "Srivatsa S. Bhat" Subject: Re: [PATCH v3 10/52] arm, kvm: Fix CPU hotplug callback registration Message-ID: <20140314191055.GF28661@cbox> References: <20140310203312.10746.310.stgit@srivatsabhat.in.ibm.com> <20140310203538.10746.25364.stgit@srivatsabhat.in.ibm.com> <20140312232127.GC24808@cbox> <53229701.8050405@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: <53229701.8050405@linux.vnet.ibm.com> Cc: ego@linux.vnet.ibm.com, kvm@vger.kernel.org, peterz@infradead.org, linux-kernel@vger.kernel.org, linuxppc-dev@ozlabs.org, paulus@samba.org, walken@google.com, kvmarm@lists.cs.columbia.edu, linux-arch@vger.kernel.org, linux@arm.linux.org.uk, mingo@kernel.org, marc.zyngier@arm.com, paulmck@linux.vnet.ibm.com, linux-pm@vger.kernel.org, Gleb Natapov , rusty@rustcorp.com.au, tglx@linutronix.de, linux-arm-kernel@lists.infradead.org, rjw@rjwysocki.net, oleg@redhat.com, tj@kernel.org, Paolo Bonzini , akpm@linux-foundation.org List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On Fri, Mar 14, 2014 at 11:13:29AM +0530, Srivatsa S. Bhat wrote: > On 03/13/2014 04:51 AM, Christoffer Dall wrote: > > On Tue, Mar 11, 2014 at 02:05:38AM +0530, Srivatsa S. Bhat wrote: > >> Subsystems that want to register CPU hotplug callbacks, as well as perform > >> initialization for the CPUs that are already online, often do it as shown > >> below: > >> > >> get_online_cpus(); > >> > >> for_each_online_cpu(cpu) > >> init_cpu(cpu); > >> > >> register_cpu_notifier(&foobar_cpu_notifier); > >> > >> put_online_cpus(); > >> > >> This is wrong, since it is prone to ABBA deadlocks involving the > >> cpu_add_remove_lock and the cpu_hotplug.lock (when running concurrently > >> with CPU hotplug operations). > >> > >> Instead, the correct and race-free way of performing the callback > >> registration is: > >> > >> cpu_notifier_register_begin(); > >> > >> for_each_online_cpu(cpu) > >> init_cpu(cpu); > >> > >> /* Note the use of the double underscored version of the API */ > >> __register_cpu_notifier(&foobar_cpu_notifier); > >> > >> cpu_notifier_register_done(); > >> > >> > >> Fix the kvm code in arm by using this latter form of callback registration. > >> > >> Cc: Christoffer Dall > >> Cc: Gleb Natapov > >> Cc: Russell King > >> Cc: Ingo Molnar > >> Cc: kvmarm@lists.cs.columbia.edu > >> Cc: kvm@vger.kernel.org > >> Cc: linux-arm-kernel@lists.infradead.org > >> Acked-by: Paolo Bonzini > >> Signed-off-by: Srivatsa S. Bhat > >> --- > >> > >> arch/arm/kvm/arm.c | 7 ++++++- > >> 1 file changed, 6 insertions(+), 1 deletion(-) > >> > >> diff --git a/arch/arm/kvm/arm.c b/arch/arm/kvm/arm.c > >> index bd18bb8..f0e50a0 100644 > >> --- a/arch/arm/kvm/arm.c > >> +++ b/arch/arm/kvm/arm.c > >> @@ -1051,21 +1051,26 @@ int kvm_arch_init(void *opaque) > >> } > >> } > >> > >> + cpu_notifier_register_begin(); > >> + > >> err = init_hyp_mode(); > >> if (err) > >> goto out_err; > >> > >> - err = register_cpu_notifier(&hyp_init_cpu_nb); > >> + err = __register_cpu_notifier(&hyp_init_cpu_nb); > >> if (err) { > >> kvm_err("Cannot register HYP init CPU notifier (%d)\n", err); > >> goto out_err; > >> } > >> > >> + cpu_notifier_register_done(); > >> + > >> hyp_cpu_pm_init(); > >> > >> kvm_coproc_table_init(); > >> return 0; > >> out_err: > >> + cpu_notifier_register_done(); > >> return err; > >> } > >> > >> > > > > Just so we're clear, the existing code was simply racy as not prone to > > deadlocks, right? > > > > This makes it clear that the test above for compatible CPUs can be quite > > easily evaded by using CPU hotplug, but we don't really have a good > > solution for handling that yet... Hmmm, grumble grumble, I guess if you > > hotplug unsupported CPUs on a KVM/ARM system for now, stuff will break. > > > > In this particular case, there was no deadlock possibility, rather the > existing code had insufficient synchronization against CPU hotplug. > > init_hyp_mode() would invoke cpu_init_hyp_mode() on currently online CPUs > using on_each_cpu(). If a CPU came online after this point and before calling > register_cpu_notifier(), that CPU would remain uninitialized because this > subsystem would miss the hot-online event. This patch fixes this bug and > also uses the new synchronization method (instead of get/put_online_cpus()) > to ensure that we don't deadlock with CPU hotplug. > Yes, that was my conclusion as well. Thanks for clarifying. (It could be noted in the commit message as well if you should feel so inclined). > > In any case: > > Acked-by: Christoffer Dall > > > > Thanks a lot! > Thanks, -Christoffer