Re: [PATCH v2] KVM: s390: add proper locking for CMMA migration bitmap

[David Hildenbrand <david@redhat.com>]

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@redhat.com>
>>>>> Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
>>>>> Cc: stable@vger.kernel.org # 4.13+
>>>>> Fixes: 190df4a212a7 (KVM: s390: CMMA tracking, ESSA emulation, migration mode)
>>>>> Reviewed-by: Claudio Imbrenda <imbrenda@linux.vnet.ibm.com>
>>>>> ---
>>>>> 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. 
> 
> 

Reviewed-by: David Hildenbrand <david@redhat.com>

-- 

Thanks,

David / dhildenb