iommu/amd: Don't hold a reference to mm_struct

With mmu_notifiers we don't need to hold a reference to the
mm_struct during the time the pasid is bound to a device. We
can rely on the .mn_release call back to inform us when the
mm_struct goes away.
Signed-off-by: Joerg Roedel <jroedel@suse.de>
Tested-by: Oded Gabbay <Oded.Gabbay@amd.com>

drivers/iommu/amd_iommu_v2.c

@@ -297,7 +297,6 @@ static void put_pasid_state_wait(struct pasid_state *pasid_state)
 		schedule();
 
 	finish_wait(&pasid_state->wq, &wait);
-	mmput(pasid_state->mm);
 	free_pasid_state(pasid_state);
 }
 
@@ -321,6 +320,13 @@ static void unbind_pasid(struct pasid_state *pasid_state)
 
 	/* Make sure no more pending faults are in the queue */
 	flush_workqueue(iommu_wq);
+
+	/*
+	 * No more faults are in the work queue and no new faults will be queued
+	 * from here on. We can safely set pasid_state->mm to NULL now as the
+	 * mm_struct might go away after we return.
+	 */
+	pasid_state->mm = NULL;
 }
 
 static void free_pasid_states_level1(struct pasid_state **tbl)
@@ -636,6 +642,7 @@ int amd_iommu_bind_pasid(struct pci_dev *pdev, int pasid,
 {
 	struct pasid_state *pasid_state;
 	struct device_state *dev_state;
+	struct mm_struct *mm;
 	u16 devid;
 	int ret;
 
@@ -659,12 +666,14 @@ int amd_iommu_bind_pasid(struct pci_dev *pdev, int pasid,
 	if (pasid_state == NULL)
 		goto out;
 
+
 	atomic_set(&pasid_state->count, 1);
 	init_waitqueue_head(&pasid_state->wq);
 	spin_lock_init(&pasid_state->lock);
 
+	mm                        = get_task_mm(task);
 	pasid_state->task         = task;
-	pasid_state->mm           = get_task_mm(task);
+	pasid_state->mm           = mm;
 	pasid_state->device_state = dev_state;
 	pasid_state->pasid        = pasid;
 	pasid_state->invalid      = false;
@@ -673,7 +682,7 @@ int amd_iommu_bind_pasid(struct pci_dev *pdev, int pasid,
 	if (pasid_state->mm == NULL)
 		goto out_free;
 
-	mmu_notifier_register(&pasid_state->mn, pasid_state->mm);
+	mmu_notifier_register(&pasid_state->mn, mm);
 
 	ret = set_pasid_state(dev_state, pasid_state, pasid);
 	if (ret)
@@ -684,16 +693,23 @@ int amd_iommu_bind_pasid(struct pci_dev *pdev, int pasid,
 	if (ret)
 		goto out_clear_state;
 
+	/*
+	 * Drop the reference to the mm_struct here. We rely on the
+	 * mmu_notifier release call-back to inform us when the mm
+	 * is going away.
+	 */
+	mmput(mm);
+
 	return 0;
 
 out_clear_state:
 	clear_pasid_state(dev_state, pasid);
 
 out_unregister:
-	mmu_notifier_unregister(&pasid_state->mn, pasid_state->mm);
+	mmu_notifier_unregister(&pasid_state->mn, mm);
 
 out_free:
-	mmput(pasid_state->mm);
+	mmput(mm);
 	free_pasid_state(pasid_state);
 
 out:
@@ -734,8 +750,18 @@ void amd_iommu_unbind_pasid(struct pci_dev *pdev, int pasid)
 	/* Clear the pasid state so that the pasid can be re-used */
 	clear_pasid_state(dev_state, pasid_state->pasid);
 
-	/* This will call the mn_release function and unbind the PASID */
-	mmu_notifier_unregister(&pasid_state->mn, pasid_state->mm);
+	/*
+	 * Check if pasid_state->mm is still valid. If mn_release has already
+	 * run it will be NULL and we can't (and don't need to) call
+	 * mmu_notifier_unregister() on it anymore.
+	 */
+	if (pasid_state->mm) {
+		/*
+		 * This will call the mn_release function and unbind
+		 * the PASID.
+		 */
+		mmu_notifier_unregister(&pasid_state->mn, pasid_state->mm);
+	}
 
 	put_pasid_state_wait(pasid_state); /* Reference taken in
 					      amd_iommu_pasid_bind */