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

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On 04/02/2015 11:59 AM, Andy Lutomirski wrote:
On Thu, Apr 2, 2015 at 11:44 AM, Radim Krčmář <rkrcmar@xxxxxxxxxx> 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@xxxxxxxxxxxxxxx
Signed-off-by: Radim Krčmář <rkrcmar@xxxxxxxxxx>
---
  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  - remove the outdated 'TODO: We can put [...]'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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