On Thu, Jun 09, 2016 at 02:16:03PM +0200, Peter Zijlstra wrote: > What's wrong with: > > u8 flags = READ_ONCE(src->flags); > > ? > > (and have the flags store be done using WRITE_ONCE() of course). > > Sure, if your total state is larger than one word you need the > seqcount for integrity, but reading _one_ byte, shees.
Something like so; although given that I've never seen this code before I can easily have missed an update site. diff --git a/arch/x86/kernel/pvclock.c b/arch/x86/kernel/pvclock.c index 99bfc025111d..7a4bf59aa929 100644 --- a/arch/x86/kernel/pvclock.c +++ b/arch/x86/kernel/pvclock.c @@ -60,15 +60,7 @@ void pvclock_resume(void) u8 pvclock_read_flags(struct pvclock_vcpu_time_info *src) { - unsigned version; - cycle_t ret; - u8 flags; - - do { - version = __pvclock_read_cycles(src, &ret, &flags); - } while ((src->version & 1) || version != src->version); - - return flags & valid_flags; + return READ_ONCE(src->flags) & valid_flags; } cycle_t pvclock_clocksource_read(struct pvclock_vcpu_time_info *src) @@ -83,7 +75,8 @@ cycle_t pvclock_clocksource_read(struct pvclock_vcpu_time_info *src) } while ((src->version & 1) || version != src->version); if (unlikely((flags & PVCLOCK_GUEST_STOPPED) != 0)) { - src->flags &= ~PVCLOCK_GUEST_STOPPED; + flags &= ~PVCLOCK_GUEST_STOPPED; + WRITE_ONCE(src->flags, flags); pvclock_touch_watchdogs(); } diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index 1ba3b7d3cae9..54458acce4f5 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -1832,7 +1832,7 @@ static int kvm_guest_time_update(struct kvm_vcpu *v) if (use_master_clock) pvclock_flags |= PVCLOCK_TSC_STABLE_BIT; - vcpu->hv_clock.flags = pvclock_flags; + WRITE_ONCE(vcpu->hv_clock.flags, pvclock_flags); trace_kvm_pvclock_update(v->vcpu_id, &vcpu->hv_clock);