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 <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.
>
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 <christoffer.dall@xxxxxxxxxx>
> >
>
> Thanks a lot!
>
Thanks,
-Christoffer
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