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.
--
Thanks,
David / dhildenb
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