Re: [PATCH] x86: vdso: fix pvclock races with task migration

[Andy Lutomirski <luto@kernel.org>]

On 04/02/2015 11:59 AM, Andy Lutomirski wrote:
> On Thu, Apr 2, 2015 at 11:44 AM, Radim Krčmář <rkrcmar@redhat.com> wrote:
>> If we were migrated right after __getcpu, but before reading the
>> migration_count, we wouldn't notice that we read TSC of a different
>> VCPU, nor that KVM's bug made pvti invalid, as only migration_count
>> on source VCPU is increased.
>>
>> Change vdso instead of updating migration_count on destination.
>
> Looks good to me.

Just to check: what tree is this intended to go through?  I can take it, 
but not until the previous patch makes it into Linus' tree or -tip.  Or 
I can take both patches.

Marcelo, Paolo?

--Andy

>
> --Andy
>
>>
>> Fixes: 0a4e6be9ca17 ("x86: kvm: Revert "remove sched notifier for cross-cpu migrations"")
>> Cc: stable@vger.kernel.org
>> Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
>> ---
>>   Because it we'll get a complete rewrite, this series does not
>>   - remove the outdated 'TODO: We can put [...]' comment
>>   - use a proper encapsulation for the inner do-while loop
>>   - optimize the outer do-while loop
>>     (no need to re-read cpu id on version mismatch)
>>
>>   arch/x86/vdso/vclock_gettime.c | 20 ++++++++++++--------
>>   1 file changed, 12 insertions(+), 8 deletions(-)
>>
>> diff --git a/arch/x86/vdso/vclock_gettime.c b/arch/x86/vdso/vclock_gettime.c
>> index 30933760ee5f..40d2473836c9 100644
>> --- a/arch/x86/vdso/vclock_gettime.c
>> +++ b/arch/x86/vdso/vclock_gettime.c
>> @@ -99,21 +99,25 @@ static notrace cycle_t vread_pvclock(int *mode)
>>                   * __getcpu() calls (Gleb).
>>                   */
>>
>> -               pvti = get_pvti(cpu);
>> +               /* Make sure migrate_count will change if we leave the VCPU. */
>> +               do {
>> +                       pvti = get_pvti(cpu);
>> +                       migrate_count = pvti->migrate_count;
>>
>> -               migrate_count = pvti->migrate_count;
>> +                       cpu1 = cpu;
>> +                       cpu = __getcpu() & VGETCPU_CPU_MASK;
>> +               } while (unlikely(cpu != cpu1));
>>
>>                  version = __pvclock_read_cycles(&pvti->pvti, &ret, &flags);
>>
>>                  /*
>>                   * Test we're still on the cpu as well as the version.
>> -                * We could have been migrated just after the first
>> -                * vgetcpu but before fetching the version, so we
>> -                * wouldn't notice a version change.
>> +                * - We must read TSC of pvti's VCPU.
>> +                * - KVM doesn't follow the versioning protocol, so data could
>> +                *   change before version if we left the VCPU.
>>                   */
>> -               cpu1 = __getcpu() & VGETCPU_CPU_MASK;
>> -       } while (unlikely(cpu != cpu1 ||
>> -                         (pvti->pvti.version & 1) ||
>> +               smp_rmb();
>> +       } while (unlikely((pvti->pvti.version & 1) ||
>>                            pvti->pvti.version != version ||
>>                            pvti->migrate_count != migrate_count));
>>
>> --
>> 2.3.4
>>
>
>
>