From patchwork Tue Mar 18 10:23:05 2014 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: "Srivatsa S. 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Violators will be prosecuted; Tue, 18 Mar 2014 20:23:29 +1000 Received: from d23relay04.au.ibm.com (d23relay04.au.ibm.com [9.190.234.120]) by d23dlp03.au.ibm.com (Postfix) with ESMTP id A19CD3578053 for ; Tue, 18 Mar 2014 21:23:28 +1100 (EST) Received: from d23av01.au.ibm.com (d23av01.au.ibm.com [9.190.234.96]) by d23relay04.au.ibm.com (8.13.8/8.13.8/NCO v10.0) with ESMTP id s2IA3IJ463963386 for ; Tue, 18 Mar 2014 21:03:19 +1100 Received: from d23av01.au.ibm.com (localhost [127.0.0.1]) by d23av01.au.ibm.com (8.14.4/8.14.4/NCO v10.0 AVout) with ESMTP id s2IANPo5020583 for ; Tue, 18 Mar 2014 21:23:27 +1100 Received: from srivatsabhat.in.ibm.com (srivatsabhat.in.ibm.com [9.124.35.74]) by d23av01.au.ibm.com (8.14.4/8.14.4/NCO v10.0 AVin) with ESMTP id s2IANEej020427; Tue, 18 Mar 2014 21:23:20 +1100 Message-ID: <53281E89.7080201@linux.vnet.ibm.com> Date: Tue, 18 Mar 2014 15:53:05 +0530 From: "Srivatsa S. Bhat" User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:15.0) Gecko/20120828 Thunderbird/15.0 MIME-Version: 1.0 To: Christoffer Dall Subject: [UPDATED PATCH v3 10/52] arm, kvm: Fix CPU hotplug callback registration 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> <20140314191055.GF28661@cbox> In-Reply-To: <20140314191055.GF28661@cbox> X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 14031810-9264-0000-0000-000005A85DF7 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20140318_062359_579343_E4F001CE X-CRM114-Status: GOOD ( 24.31 ) X-Spam-Score: -4.2 (----) Cc: ego@linux.vnet.ibm.com, kvm@vger.kernel.org, peterz@infradead.org, oleg@redhat.com, linuxppc-dev@ozlabs.org, paulus@samba.org, walken@google.com, mingo@kernel.org, linux-arch@vger.kernel.org, linux@arm.linux.org.uk, kvmarm@lists.cs.columbia.edu, Gleb Natapov , paulmck@linux.vnet.ibm.com, linux-pm@vger.kernel.org, marc.zyngier@arm.com, rusty@rustcorp.com.au, tglx@linutronix.de, linux-arm-kernel@lists.infradead.org, rjw@rjwysocki.net, linux-kernel@vger.kernel.org, tj@kernel.org, Paolo Bonzini , akpm@linux-foundation.org X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.15 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+patchwork-linux-arm=patchwork.kernel.org@lists.infradead.org X-Spam-Status: No, score=-4.2 required=5.0 tests=BAYES_00, RCVD_IN_DNSWL_MED, T_RP_MATCHES_RCVD, UNPARSEABLE_RELAY autolearn=unavailable version=3.3.1 X-Spam-Checker-Version: SpamAssassin 3.3.1 (2010-03-16) on mail.kernel.org X-Virus-Scanned: ClamAV using ClamSMTP On 03/15/2014 12:40 AM, Christoffer Dall wrote: > 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: >>>> [...] >>> 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). > Please find the patch with updated changelog (and your Ack) below. (No changes in code). From: Srivatsa S. Bhat Subject: [PATCH] arm, kvm: Fix CPU hotplug callback registration 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(); In the existing arm kvm code, there is no synchronization with CPU hotplug to avoid missing the hotplug events that might occur after invoking init_hyp_mode() and before calling register_cpu_notifier(). Fix this bug and also use the new synchronization method (instead of get/put_online_cpus()) to ensure that we don't deadlock with CPU hotplug. 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 Acked-by: Christoffer Dall 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; }