Re: x86: kvm: Revert "remove sched notifier for cross-cpu migrations"

["Radim Krčmář" <rkrcmar@redhat.com>]

2015-03-23 20:21-0300, Marcelo Tosatti:
> 
> The following point:
> 
>     2. per-CPU pvclock time info is updated if the
>        underlying CPU changes.
> 
> Is not true anymore since "KVM: x86: update pvclock area conditionally,
> on cpu migration".
> 
> Add task migration notification back.
> 
> Problem noticed by Andy Lutomirski.
> 
> Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
> CC: stable@kernel.org # 3.11+

Revert contains a bug that got pointed out in the discussion:

> diff --git a/arch/x86/vdso/vclock_gettime.c b/arch/x86/vdso/vclock_gettime.c
>  	do {
>  		cpu = __getcpu() & VGETCPU_CPU_MASK;
>  
>  		pvti = get_pvti(cpu);

We can migrate to 'other cpu' here.

> +		migrate_count = pvti->migrate_count;
> +
>  		version = __pvclock_read_cycles(&pvti->pvti, &ret, &flags);

And migrate back to 'cpu' here.

rdtsc was executed on different cpu, so pvti and tsc might not be in
sync, but migrate_count hasn't changed.

>  		cpu1 = __getcpu() & VGETCPU_CPU_MASK;

(Reading cpuid here is useless.)

>  	} while (unlikely(cpu != cpu1 ||
>  			  (pvti->pvti.version & 1) ||
> -			  pvti->pvti.version != version));
> +			  pvti->pvti.version != version ||
> +			  pvti->migrate_count != migrate_count));

We can workaround the bug with,

  	cpu = __getcpu() & VGETCPU_CPU_MASK;
  	pvti = get_pvti(cpu);
  	migrate_count = pvti->migrate_count;
  	if (cpu != (__getcpu() & VGETCPU_CPU_MASK))
  		continue;