| Message ID | 53281E89.7080201@linux.vnet.ibm.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On 18 March 2014 03:23, Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com> wrote: > 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). > perfect, thanks! > From: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com> > 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 <gleb@kernel.org> > Cc: Russell King <linux@arm.linux.org.uk> > Cc: Ingo Molnar <mingo@kernel.org> > Cc: kvmarm@lists.cs.columbia.edu > Cc: kvm@vger.kernel.org > Cc: linux-arm-kernel@lists.infradead.org > Acked-by: Paolo Bonzini <pbonzini@redhat.com> > Acked-by: Christoffer Dall <christoffer.dall@linaro.org> > Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com> > --- > > 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; > } > > >
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; }