On 01/24/2018 02:05 PM, David Hildenbrand wrote:
> On 24.01.2018 14:02, Christian Borntraeger wrote:
>>
>>
>> On 01/24/2018 01:54 PM, David Hildenbrand wrote:
>>> On 24.01.2018 13:51, Christian Borntraeger wrote:
>>>>
>>>>
>>>> On 01/24/2018 01:51 PM, David Hildenbrand wrote:
>>>>> On 24.01.2018 13:41, Christian Borntraeger wrote:
>>>>>> Some parts of the cmma migration bitmap is already protected
>>>>>> with the kvm->lock (e.g. the migration start). On the other
>>>>>> hand the read of the cmma bits is not protected against a
>>>>>> concurrent free, neither is the emulation of the ESSA instruction.
>>>>>> Let's extend the locking to all related ioctls by using
>>>>>> the slots lock and wait for the freeing until all unlocked
>>>>>> users have finished (those hold kvm->srcu for read).
>>>>>>
>>>>>> Reported-by: David Hildenbrand <david@xxxxxxxxxx>
>>>>>> Signed-off-by: Christian Borntraeger <borntraeger@xxxxxxxxxx>
>>>>>> Cc: stable@xxxxxxxxxxxxxxx # 4.13+
>>>>>> Fixes: 190df4a212a7 (KVM: s390: CMMA tracking, ESSA emulation, migration mode)
>>>>>> Reviewed-by: Claudio Imbrenda <imbrenda@xxxxxxxxxxxxxxxxxx>
>>>>>> ---
>>>>>> v1->v2: fix comments in kvm_s390_vm_[start|stop]_migration
>>>>>> arch/s390/kvm/kvm-s390.c | 18 +++++++++++-------
>>>>>> 1 file changed, 11 insertions(+), 7 deletions(-)
>>>>>>
>>>>>> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
>>>>>> index 2a5a9f0617f88..e81b748af4553 100644
>>>>>> --- a/arch/s390/kvm/kvm-s390.c
>>>>>> +++ b/arch/s390/kvm/kvm-s390.c
>>>>>> @@ -769,7 +769,7 @@ static void kvm_s390_sync_request_broadcast(struct kvm *kvm, int req)
>>>>>>
>>>>>> /*
>>>>>> * Must be called with kvm->srcu held to avoid races on memslots, and with
>>>>>> - * kvm->lock to avoid races with ourselves and kvm_s390_vm_stop_migration.
>>>>>> + * kvm->slots_lock to avoid races with ourselves and kvm_s390_vm_stop_migration.
>>>>>> */
>>>>>> static int kvm_s390_vm_start_migration(struct kvm *kvm)
>>>>>> {
>>>>>> @@ -824,7 +824,7 @@ static int kvm_s390_vm_start_migration(struct kvm *kvm)
>>>>>> }
>>>>>>
>>>>>> /*
>>>>>> - * Must be called with kvm->lock to avoid races with ourselves and
>>>>>> + * Must be called with kvm->slots_lock to avoid races with ourselves and
>>>>>> * kvm_s390_vm_start_migration.
>>>>>> */
>>>>>> static int kvm_s390_vm_stop_migration(struct kvm *kvm)
>>>>>> @@ -839,6 +839,8 @@ static int kvm_s390_vm_stop_migration(struct kvm *kvm)
>>>>>>
>>>>>> if (kvm->arch.use_cmma) {
>>>>>> kvm_s390_sync_request_broadcast(kvm, KVM_REQ_STOP_MIGRATION);
>>>>>> + /* We have to wait for the essa emulation to finish */
>>>>>> + synchronize_srcu(&kvm->srcu);
>>>>>> vfree(mgs->pgste_bitmap);
>>>>>> }
>>>>>> kfree(mgs);
>>>>>> @@ -848,14 +850,12 @@ static int kvm_s390_vm_stop_migration(struct kvm *kvm)
>>>>>> static int kvm_s390_vm_set_migration(struct kvm *kvm,
>>>>>> struct kvm_device_attr *attr)
>>>>>> {
>>>>>> - int idx, res = -ENXIO;
>>>>>> + int res = -ENXIO;
>>>>>>
>>>>>> - mutex_lock(&kvm->lock);
>>>>>> + mutex_lock(&kvm->slots_lock);
>>>>>
>>>>> Why were we holding the kvm lock here? Is it okay to drop this lock now?
>>>>>
>>>>> (I assume it only existed to avoid having multiple concurrent users
>>>>> trying to set the migration state - in that case this is fine)
>>>>
>>>> Yes, it was used to prevent multiple users.
>>>
>>> So we are using srcu to protect a pointer not managed via rcu. Looks
>>> strange, but guess this works because of the request bit handling.
>>
>> No we are using kvm_slots_locks for all migration related functions:
>> -kvm_s390_vm_start_migration
>> -kvm_s390_vm_stop_migration
>> -kvm_s390_set_cmma_bits
>> -kvm_s390_get_cmma_bits
>>
>>
>> In addition to that we use synchronize_srcu to ensure that the essa handler
>> has finished.
I think I will add that to the patch description.
>>
>>
>
> Reviewed-by: David Hildenbrand <david@xxxxxxxxxx>
>
