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.
