Messages in this thread | | | Date | Fri, 14 Mar 2014 11:13:29 +0530 | From | "Srivatsa S. Bhat" <> | Subject | Re: [PATCH v3 10/52] arm, kvm: Fix CPU hotplug callback registration |
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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 <christoffer.dall@linaro.org> >> 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> >> 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; >> } >> >> > > 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.
> In any case: > Acked-by: Christoffer Dall <christoffer.dall@linaro.org> >
Thanks a lot!
Regards, Srivatsa S. Bhat
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