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@xxxxxxxxxx>
>> Cc: Gleb Natapov <gleb@xxxxxxxxxx>
>> Cc: Russell King <linux@xxxxxxxxxxxxxxxx>
>> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
>> Cc: kvmarm@xxxxxxxxxxxxxxxxxxxxx
>> Cc: kvm@xxxxxxxxxxxxxxx
>> Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
>> Acked-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
>> Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@xxxxxxxxxxxxxxxxxx>
>> ---
>>
>> 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@xxxxxxxxxx>
>
Thanks a lot!
Regards,
Srivatsa S. Bhat
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