From 1365039d0cb32c0cf96eb9f750f4277c9a90f87d Mon Sep 17 00:00:00 2001 From: Christian Borntraeger Date: Tue, 4 Nov 2014 08:31:16 +0100 Subject: [PATCH] KVM: s390: Fix ipte locking ipte_unlock_siif uses cmpxchg to replace the in-memory data of the ipte lock together with ACCESS_ONCE for the intial read. union ipte_control { unsigned long val; struct { unsigned long k : 1; unsigned long kh : 31; unsigned long kg : 32; }; }; [...] static void ipte_unlock_siif(struct kvm_vcpu *vcpu) { union ipte_control old, new, *ic; ic = &vcpu->kvm->arch.sca->ipte_control; do { new = old = ACCESS_ONCE(*ic); new.kh--; if (!new.kh) new.k = 0; } while (cmpxchg(&ic->val, old.val, new.val) != old.val); if (!new.kh) wake_up(&vcpu->kvm->arch.ipte_wq); } The new value, is loaded twice from memory with gcc 4.7.2 of fedora 18, despite the ACCESS_ONCE: ---> l %r4,0(%r3) <--- load first 32 bit of lock (k and kh) in r4 alfi %r4,2147483647 <--- add -1 to r4 llgtr %r4,%r4 <--- zero out the sign bit of r4 lg %r1,0(%r3) <--- load all 64 bit of lock into new lgr %r2,%r1 <--- load the same into old risbg %r1,%r4,1,31,32 <--- shift and insert r4 into the bits 1-31 of new llihf %r4,2147483647 ngrk %r4,%r1,%r4 jne aa0 nihh %r1,32767 lgr %r4,%r2 csg %r4,%r1,0(%r3) cgr %r2,%r4 jne a70 If the memory value changes between the first load (l) and the second load (lg) we are broken. If that happens VCPU threads will hang (unkillable) in handle_ipte_interlock. Andreas Krebbel analyzed this and tracked it down to a compiler bug in that version: "while it is not that obvious the C99 standard basically forbids duplicating the memory access also in that case. For an argumentation of a similiar case please see: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=22278#c43 For the implementation-defined cases regarding volatile there are some GCC-specific clarifications which can be found here: https://gcc.gnu.org/onlinedocs/gcc/Volatiles.html#Volatiles I've tracked down the problem with a reduced testcase. The problem was that during a tree level optimization (SRA - scalar replacement of aggregates) the volatile marker is lost. And an RTL level optimizer (CSE - common subexpression elimination) then propagated the memory read into its second use introducing another access to the memory location. So indeed Christian's suspicion that the union access has something to do with it is correct (since it triggered the SRA optimization). This issue has been reported and fixed in the GCC 4.8 development cycle: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58145" This patch replaces the ACCESS_ONCE scheme with a barrier() based scheme that should work for all supported compilers. Signed-off-by: Christian Borntraeger Cc: stable@vger.kernel.org # v3.16+ --- arch/s390/kvm/gaccess.c | 20 ++++++++++++++------ 1 file changed, 14 insertions(+), 6 deletions(-) diff --git a/arch/s390/kvm/gaccess.c b/arch/s390/kvm/gaccess.c index 0f961a1c64b3..6dc0ad9c7050 100644 --- a/arch/s390/kvm/gaccess.c +++ b/arch/s390/kvm/gaccess.c @@ -229,10 +229,12 @@ static void ipte_lock_simple(struct kvm_vcpu *vcpu) goto out; ic = &vcpu->kvm->arch.sca->ipte_control; do { - old = ACCESS_ONCE(*ic); + old = *ic; + barrier(); while (old.k) { cond_resched(); - old = ACCESS_ONCE(*ic); + old = *ic; + barrier(); } new = old; new.k = 1; @@ -251,7 +253,9 @@ static void ipte_unlock_simple(struct kvm_vcpu *vcpu) goto out; ic = &vcpu->kvm->arch.sca->ipte_control; do { - new = old = ACCESS_ONCE(*ic); + old = *ic; + barrier(); + new = old; new.k = 0; } while (cmpxchg(&ic->val, old.val, new.val) != old.val); wake_up(&vcpu->kvm->arch.ipte_wq); @@ -265,10 +269,12 @@ static void ipte_lock_siif(struct kvm_vcpu *vcpu) ic = &vcpu->kvm->arch.sca->ipte_control; do { - old = ACCESS_ONCE(*ic); + old = *ic; + barrier(); while (old.kg) { cond_resched(); - old = ACCESS_ONCE(*ic); + old = *ic; + barrier(); } new = old; new.k = 1; @@ -282,7 +288,9 @@ static void ipte_unlock_siif(struct kvm_vcpu *vcpu) ic = &vcpu->kvm->arch.sca->ipte_control; do { - new = old = ACCESS_ONCE(*ic); + old = *ic; + barrier(); + new = old; new.kh--; if (!new.kh) new.k = 0;