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

[Marcelo Tosatti <mtosatti@redhat.com>]

On Thu, Apr 02, 2015 at 08:44:23PM +0200, Radim Krčmář 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.
> 
> 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  - remove the outdated 'TODO: We can put [...]'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
> 
> --
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Reviewed-by: Marcelo Tosatti <mtosatti@redhat.com>