On Thu, Nov 7, 2013 at 5:54 PM, Alexander Graf <agraf@xxxxxxx> wrote:
>
> On 07.11.2013, at 07:22, Liu Ping Fan <kernelfans@xxxxxxxxx> wrote:
>
>> Since kvmppc_hv_find_lock_hpte() is called from both virtmode and
>> realmode, so it can trigger the deadlock.
>>
>> Suppose the following scene:
>>
>> Two physical cpuM, cpuN, two VM instances A, B, each VM has a group of vcpus.
>>
>> If on cpuM, vcpu_A_1 holds bitlock X (HPTE_V_HVLOCK), then is switched out,
>> and on cpuN, vcpu_A_2 try to lock X in realmode, then cpuN will be caught in
>> realmode for a long time.
>>
>> What makes things even worse if the following happens,
>> On cpuM, bitlockX is hold, on cpuN, Y is hold.
>> vcpu_B_2 try to lock Y on cpuM in realmode
>> vcpu_A_2 try to lock X on cpuN in realmode
>>
>> Oops! deadlock happens
>>
>> Signed-off-by: Liu Ping Fan <pingfank@xxxxxxxxxxxxxxxxxx>
>
> Very nice catch :).
>
> I think it makes a lot of sense to document the fact that kvmppc_hv_find_lock_hpte() should be called with preemption disabled in a comment above the function, so that next time when someone potentially calls it, he knows that he needs to put preempt_disable() around it.
>
Ok, I will document them in v3
> Thanks a lot for finding this pretty subtle issue. May I ask how you got there? Did you actually see systems deadlock because of this?
>
Intuition :). then I begin try to model a scene which causes the
deadlock. And fortunately, I find a case to verify my suspension.
Regards,
Pingfan
>
> Alex
>
>> ---
>> arch/powerpc/kvm/book3s_64_mmu_hv.c | 6 +++++-
>> 1 file changed, 5 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/powerpc/kvm/book3s_64_mmu_hv.c b/arch/powerpc/kvm/book3s_64_mmu_hv.c
>> index 043eec8..dbc1478 100644
>> --- a/arch/powerpc/kvm/book3s_64_mmu_hv.c
>> +++ b/arch/powerpc/kvm/book3s_64_mmu_hv.c
>> @@ -474,10 +474,13 @@ static int kvmppc_mmu_book3s_64_hv_xlate(struct kvm_vcpu *vcpu, gva_t eaddr,
>> }
>>
>> /* Find the HPTE in the hash table */
>> + preempt_disable();
>> index = kvmppc_hv_find_lock_hpte(kvm, eaddr, slb_v,
>> HPTE_V_VALID | HPTE_V_ABSENT);
>> - if (index < 0)
>> + if (index < 0) {
>> + preempt_enable();
>> return -ENOENT;
>> + }
>> hptep = (unsigned long *)(kvm->arch.hpt_virt + (index << 4));
>> v = hptep[0] & ~HPTE_V_HVLOCK;
>> gr = kvm->arch.revmap[index].guest_rpte;
>> @@ -485,6 +488,7 @@ static int kvmppc_mmu_book3s_64_hv_xlate(struct kvm_vcpu *vcpu, gva_t eaddr,
>> /* Unlock the HPTE */
>> asm volatile("lwsync" : : : "memory");
>> hptep[0] = v;
>> + preempt_enable();
>>
>> gpte->eaddr = eaddr;
>> gpte->vpage = ((v & HPTE_V_AVPN) << 4) | ((eaddr >> 12) & 0xfff);
>> --
>> 1.8.1.4
>>
>
--
To unsubscribe from this list: send the line "unsubscribe kvm-ppc" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
