Re: [PATCH] static_key: fix concurrent static_key_slow_inc

[Paolo Bonzini <pbonzini@redhat.com>]

On 22/06/2016 10:50, Christian Borntraeger wrote:
> On 06/21/2016 06:52 PM, Paolo Bonzini wrote:
>> The following scenario is possible:
>>
>>     CPU 1                                   CPU 2
>>     static_key_slow_inc
>>      atomic_inc_not_zero
>>       -> key.enabled == 0, no increment
>>      jump_label_lock
>>      atomic_inc_return
>>       -> key.enabled == 1 now
>>                                             static_key_slow_inc
>>                                              atomic_inc_not_zero
>>                                               -> key.enabled == 1, inc to 2
>>                                              return
>>                                             ** static key is wrong!
>>      jump_label_update
>>      jump_label_unlock
>>
>> Testing the static key at the point marked by (**) will follow the wrong
>> path for jumps that have not been patched yet.  This can actually happen
>> when creating many KVM virtual machines with userspace LAPIC emulation;
>> just run several copies of the following program:
>>
>>     #include <fcntl.h>
>>     #include <unistd.h>
>>     #include <sys/ioctl.h>
>>     #include <linux/kvm.h>
>>
>>     int main(void)
>>     {
>>         for (;;) {
>>             int kvmfd = open("/dev/kvm", O_RDONLY);
>>             int vmfd = ioctl(kvmfd, KVM_CREATE_VM, 0);
>>             close(ioctl(vmfd, KVM_CREATE_VCPU, 1));
>>             close(vmfd);
>>             close(kvmfd);
>>         }
>>         return 0;
>>     }
>>
>> Every KVM_CREATE_VCPU ioctl will attempt a static_key_slow_inc.  The
>> static key's purpose is to skip NULL pointer checks and indeed one of
>> the processes eventually dereferences NULL.
> 
> Interesting. Some time ago I had a spurious bug on the preempt_notifier
> when starting/stopping lots of guests, but I was never able to reliably 
> reproduce it. I was chasing some other bug, so I did not even considered
> static_key to be broken, but this might actually be the fix for that
> problem.

It could be the same that was reported here:
http://article.gmane.org/gmane.comp.emulators.kvm.devel/154069

Paolo