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

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

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);
-- 
1.9.1