[PATCH] drm/amdkfd: Fix circular lock in nocpsch path

[Felix Kuehling]

Am 2021-06-16 um 12:01 a.m. schrieb Pan, Xinhui:
>> 2021年6月16日 02:22，Kuehling, Felix <Felix.Kuehling at amd.com> 写道：
>>
>> [+Xinhui]
>>
>>
>> Am 2021-06-15 um 1:50 p.m. schrieb Amber Lin:
>>> Calling free_mqd inside of destroy_queue_nocpsch_locked can cause a
>>> circular lock. destroy_queue_nocpsch_locked is called under a DQM lock,
>>> which is taken in MMU notifiers, potentially in FS reclaim context.
>>> Taking another lock, which is BO reservation lock from free_mqd, while
>>> causing an FS reclaim inside the DQM lock creates a problematic circular
>>> lock dependency. Therefore move free_mqd out of
>>> destroy_queue_nocpsch_locked and call it after unlocking DQM.
>>>
>>> Signed-off-by: Amber Lin <Amber.Lin at amd.com>
>>> Reviewed-by: Felix Kuehling <Felix.Kuehling at amd.com>
>> Let's submit this patch as is. I'm making some comments inline for
>> things that Xinhui can address in his race condition patch.
>>
>>
>>> ---
>>> .../drm/amd/amdkfd/kfd_device_queue_manager.c  | 18 +++++++++++++-----
>>> 1 file changed, 13 insertions(+), 5 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/amd/amdkfd/kfd_device_queue_manager.c b/drivers/gpu/drm/amd/amdkfd/kfd_device_queue_manager.c
>>> index 72bea5278add..c069fa259b30 100644
>>> --- a/drivers/gpu/drm/amd/amdkfd/kfd_device_queue_manager.c
>>> +++ b/drivers/gpu/drm/amd/amdkfd/kfd_device_queue_manager.c
>>> @@ -486,9 +486,6 @@ static int destroy_queue_nocpsch_locked(struct device_queue_manager *dqm,
>>> 	if (retval == -ETIME)
>>> 		qpd->reset_wavefronts = true;
>>>
>>> -
>>> -	mqd_mgr->free_mqd(mqd_mgr, q->mqd, q->mqd_mem_obj);
>>> -
>>> 	list_del(&q->list);
>>> 	if (list_empty(&qpd->queues_list)) {
>>> 		if (qpd->reset_wavefronts) {
>>> @@ -523,6 +520,8 @@ static int destroy_queue_nocpsch(struct device_queue_manager *dqm,
>>> 	int retval;
>>> 	uint64_t sdma_val = 0;
>>> 	struct kfd_process_device *pdd = qpd_to_pdd(qpd);
>>> +	struct mqd_manager *mqd_mgr =
>>> +		dqm->mqd_mgrs[get_mqd_type_from_queue_type(q->properties.type)];
>>>
>>> 	/* Get the SDMA queue stats */
>>> 	if ((q->properties.type == KFD_QUEUE_TYPE_SDMA) ||
>>> @@ -540,6 +539,8 @@ static int destroy_queue_nocpsch(struct device_queue_manager *dqm,
>>> 		pdd->sdma_past_activity_counter += sdma_val;
>>> 	dqm_unlock(dqm);
>>>
>>> +	mqd_mgr->free_mqd(mqd_mgr, q->mqd, q->mqd_mem_obj);
>>> +
>>> 	return retval;
>>> }
>>>
>>> @@ -1629,7 +1630,7 @@ static bool set_cache_memory_policy(struct device_queue_manager *dqm,
>>> static int process_termination_nocpsch(struct device_queue_manager *dqm,
>>> 		struct qcm_process_device *qpd)
>>> {
>>> -	struct queue *q, *next;
>>> +	struct queue *q;
>>> 	struct device_process_node *cur, *next_dpn;
>>> 	int retval = 0;
>>> 	bool found = false;
>>> @@ -1637,12 +1638,19 @@ static int process_termination_nocpsch(struct device_queue_manager *dqm,
>>> 	dqm_lock(dqm);
>>>
>>> 	/* Clear all user mode queues */
>>> -	list_for_each_entry_safe(q, next, &qpd->queues_list, list) {
>>> +	while (!list_empty(&qpd->queues_list)) {
>>> +		struct mqd_manager *mqd_mgr;
>>> 		int ret;
>>>
>>> +		q = list_first_entry(&qpd->queues_list, struct queue, list);
>>> +		mqd_mgr = dqm->mqd_mgrs[get_mqd_type_from_queue_type(
>>> +				q->properties.type)];
>>> 		ret = destroy_queue_nocpsch_locked(dqm, qpd, q);
>>> 		if (ret)
>>> 			retval = ret;
>>> +		dqm_unlock(dqm);
>>> +		mqd_mgr->free_mqd(mqd_mgr, q->mqd, q->mqd_mem_obj);
>>> +		dqm_lock(dqm);
>> This is the correct way to clean up the list when dropping the dqm-lock
>> in the middle. Xinhui, you can use the same method in
>> process_termination_cpsch.
>>
> yes, that is the right way to walk through the list. thanks.
>
>
>> I believe the swapping of the q->mqd with a temporary variable is not
>> needed. When free_mqd is called, the queue is no longer on the
>> qpd->queues_list, so destroy_queue cannot race with it. If we ensure
>> that queues are always removed from the list before calling free_mqd,
>> and that list-removal happens under the dqm_lock, then there should be
>> no risk of a race condition that causes a double-free.
>>
> no, the double free exists because pqm_destroy_queue fetch the queue from qid by get_queue_by_qid()
> the race is like below.
> pqm_destroy_queue
> 	get_queue_by_qid				process_termination_cpsch
> 	destroy_queue_cpsch
> 								lock
> 								list_for_each_entry_safe
> 									list_del(q)
> 								unlock
> 								free_mqd
> 	lock
> 	list_del(q)
> 	unlock
> 	free_mqd

I think if both those threads try to free the same queue, they both need
to hold the same process->mutex. For pqm_destroy_queue that happens in
kfd_ioctl_destroy_queue. For process_termination_cpsch that happens in
kfd_process_notifier_release before it calls
kfd_process_dequeue_from_all_devices.

Regards,
  Felix


> 	
>
>
>  
>> Regards,
>>   Felix
>>
>>
>>> 	}
>>>
>>> 	/* Unregister process */