Re: [PATCH 1/1] iommu/amd: Use delayed mmu release notifier

[Oded Gabbay <oded.gabbay@amd.com>]

Hi Joerg,

Could you please review this patch ?

Thanks,

	Oded

On 10/25/2014 10:16 PM, Oded Gabbay wrote:
> Hi,
> Could anyone please review this patch ?
>
> Thanks,
> 	Oded
>
> On 18/10/14 00:43, Oded Gabbay wrote:
>> This patch makes use of the new delayed mmu release notifier feature in
>> mm code. This is necessary because on the one hand amd_iommu_unbind_pasid
>> must be called explicitly during the tear-down of a process, but on the
>> other hand, it could be called from a function (e.g. in amdkfd)
>> which is a call-back function for the mmu notifier release.
>> In such a case, amd_iommu_unbind_pasid must not free the pasid_state
>> object, as it is a member in the list of mmu release notifiers (and
>> freeing it in the middle of iterating the list will break the list).
>>
>> Therefore, this patch delays the release of pasid_state to a later
>> call-back, which is called inside an srcu, and there we can freely
>> release the object.
>>
>> The flow of function calls when a process is teared-down looks like this:
>> (This flow assumes that amdkfd is the client of amd_iommu_v2)
>>
>> 1. mmu release notifiers for the destroyed process are started to get called.
>>
>> 2. amd_iommu_v2 notifier gets called, and it calls a call-back
>>     function (inv_ctx_cb). amdkfd, which implements this call-back function,
>>     performs tear-down of the relevant queues per device per process.
>>
>> 3. Later, amdkfd's mmu notifier callback (kfd_process_notifier_release()) gets
>>     called and releases more things that are related to the process.
>>     In that function, amd_iommu_unbind_pasid() is explicitly called.
>>
>> 4. (current code) amd_iommu_unbind_pasid() frees the mmu notifier
>>     object itself, which mustn't be freed while iterating over the list
>>     of mmu notifiers.
>>
>> 4. (new code in this patch) amd_iommu_unbind_pasid() sets a delayed notifier,
>>     using the delayed mmu release notifier feature (new in 3.17),
>>     which does the actual release later, after the iteration over the list of
>>     mmu notifiers is over.
>>
>> Signed-off-by: Oded Gabbay <oded.gabbay@amd.com>
>> ---
>>   drivers/iommu/amd_iommu_v2.c | 32 +++++++++++++++++++++++++++++---
>>   1 file changed, 29 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/iommu/amd_iommu_v2.c b/drivers/iommu/amd_iommu_v2.c
>> index 5f578e8..1e83bdd 100644
>> --- a/drivers/iommu/amd_iommu_v2.c
>> +++ b/drivers/iommu/amd_iommu_v2.c
>> @@ -57,6 +57,9 @@ struct pasid_state {
>>   	spinlock_t lock;			/* Protect pri_queues and
>>   						   mmu_notifer_count */
>>   	wait_queue_head_t wq;			/* To wait for count == 0 */
>> +
>> +	struct rcu_head	rcu;			/* Use for delayed freeing of
>> +						   pasid_state structure */
>>   };
>>
>>   struct device_state {
>> @@ -297,7 +300,6 @@ static void put_pasid_state_wait(struct pasid_state *pasid_state)
>>   		schedule();
>>
>>   	finish_wait(&pasid_state->wq, &wait);
>> -	free_pasid_state(pasid_state);
>>   }
>>
>>   static void unbind_pasid(struct pasid_state *pasid_state)
>> @@ -369,6 +371,8 @@ static void free_pasid_states(struct device_state *dev_state)
>>   		put_pasid_state_wait(pasid_state); /* Reference taken in
>>   						      amd_iommu_bind_pasid */
>>
>> +		free_pasid_state(pasid_state);
>> +
>>   		/* Drop reference taken in amd_iommu_bind_pasid */
>>   		put_device_state(dev_state);
>>   	}
>> @@ -711,6 +715,17 @@ out:
>>   }
>>   EXPORT_SYMBOL(amd_iommu_bind_pasid);
>>
>> +static void pasid_state_destroy_delayed(struct rcu_head *rcu)
>> +{
>> +	struct pasid_state *pasid_state;
>> +
>> +	pasid_state = container_of(rcu, struct pasid_state, rcu);
>> +
>> +	mmdrop(pasid_state->mm);
>> +
>> +	free_pasid_state(pasid_state);
>> +}
>> +
>>   void amd_iommu_unbind_pasid(struct pci_dev *pdev, int pasid)
>>   {
>>   	struct pasid_state *pasid_state;
>> @@ -743,13 +758,24 @@ void amd_iommu_unbind_pasid(struct pci_dev *pdev, int pasid)
>>   	clear_pasid_state(dev_state, pasid_state->pasid);
>>
>>   	/*
>> +	 * Because we drop mm_count inside pasid_state_destroy_delayed
>> +	 * and because the mmu_notifier_unregister function also drop
>> +	 * mm_count we need to take an extra count here.
>> +	 */
>> +	atomic_inc(&pasid_state->mm->mm_count);
>> +
>> +	/*
>>   	 * Call mmu_notifier_unregister to drop our reference
>>   	 * to pasid_state->mm
>>   	 */
>> -	mmu_notifier_unregister(&pasid_state->mn, pasid_state->mm);
>> +	mmu_notifier_unregister_no_release(&pasid_state->mn, pasid_state->mm);
>>
>>   	put_pasid_state_wait(pasid_state); /* Reference taken in
>> -					      amd_iommu_bind_pasid */
>> +				      amd_iommu_pasid_bind */
>> +
>> +	mmu_notifier_call_srcu(&pasid_state->rcu,
>> +				&pasid_state_destroy_delayed);
>> +
>>   out:
>>   	/* Drop reference taken in this function */
>>   	put_device_state(dev_state);
>>
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