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Message-ID: <5522E789.6070804@kernel.org>
Date: Mon, 06 Apr 2015 13:07:37 -0700
From: Andy Lutomirski <luto@...nel.org>
To: Radim Krčmář <rkrcmar@...hat.com>
CC: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
kvm list <kvm@...r.kernel.org>,
Paolo Bonzini <pbonzini@...hat.com>,
Marcelo Tosatti <mtosatti@...hat.com>,
stable <stable@...r.kernel.org>
Subject: Re: [PATCH] x86: vdso: fix pvclock races with task migration
On 04/02/2015 11:59 AM, Andy Lutomirski wrote:
> On Thu, Apr 2, 2015 at 11:44 AM, Radim Krčmář <rkrcmar@...hat.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@...r.kernel.org
>> Signed-off-by: Radim Krčmář <rkrcmar@...hat.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 - remove the outdated 'TODO: We can put [...]'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
>>
>
>
>
--
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