drm/amdkfd: Fixed kfd_process cleanup on module exit.

Handle case when module is unloaded (kfd_exit) before a process space
(mm_struct) is released.

v2: Fixed potential race conditions by removing all kfd_process from
the process table first, then working on releasing the resources.

v3: Fixed loop element access / synchronization.  Fixed extra empty lines.
Signed-off-by: David Belanger <david.belanger@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdkfd/kfd_module.c

@@ -77,6 +77,7 @@ static int kfd_init(void)
 
 static void kfd_exit(void)
 {
+	kfd_cleanup_processes();
 	kfd_debugfs_fini();
 	kfd_process_destroy_wq();
 	kfd_procfs_shutdown();

drivers/gpu/drm/amd/amdkfd/kfd_priv.h

@@ -928,6 +928,7 @@ bool kfd_dev_is_large_bar(struct kfd_dev *dev);
 
 int kfd_process_create_wq(void);
 void kfd_process_destroy_wq(void);
+void kfd_cleanup_processes(void);
 struct kfd_process *kfd_create_process(struct file *filep);
 struct kfd_process *kfd_get_process(const struct task_struct *task);
 struct kfd_process *kfd_lookup_process_by_pasid(u32 pasid);

drivers/gpu/drm/amd/amdkfd/kfd_process.c

@@ -1167,6 +1167,17 @@ static void kfd_process_free_notifier(struct mmu_notifier *mn)
 	kfd_unref_process(container_of(mn, struct kfd_process, mmu_notifier));
 }
 
+static void kfd_process_notifier_release_internal(struct kfd_process *p)
+{
+	cancel_delayed_work_sync(&p->eviction_work);
+	cancel_delayed_work_sync(&p->restore_work);
+
+	/* Indicate to other users that MM is no longer valid */
+	p->mm = NULL;
+
+	mmu_notifier_put(&p->mmu_notifier);
+}
+
 static void kfd_process_notifier_release(struct mmu_notifier *mn,
 					struct mm_struct *mm)
 {
@@ -1181,17 +1192,22 @@ static void kfd_process_notifier_release(struct mmu_notifier *mn,
 		return;
 
 	mutex_lock(&kfd_processes_mutex);
+	/*
+	 * Do early return if table is empty.
+	 *
+	 * This could potentially happen if this function is called concurrently
+	 * by mmu_notifier and by kfd_cleanup_pocesses.
+	 *
+	 */
+	if (hash_empty(kfd_processes_table)) {
+		mutex_unlock(&kfd_processes_mutex);
+		return;
+	}
 	hash_del_rcu(&p->kfd_processes);
 	mutex_unlock(&kfd_processes_mutex);
 	synchronize_srcu(&kfd_processes_srcu);
 
-	cancel_delayed_work_sync(&p->eviction_work);
-	cancel_delayed_work_sync(&p->restore_work);
-
-	/* Indicate to other users that MM is no longer valid */
-	p->mm = NULL;
-
-	mmu_notifier_put(&p->mmu_notifier);
+	kfd_process_notifier_release_internal(p);
 }
 
 static const struct mmu_notifier_ops kfd_process_mmu_notifier_ops = {
@@ -1200,6 +1216,43 @@ static const struct mmu_notifier_ops kfd_process_mmu_notifier_ops = {
 	.free_notifier = kfd_process_free_notifier,
 };
 
+/*
+ * This code handles the case when driver is being unloaded before all
+ * mm_struct are released.  We need to safely free the kfd_process and
+ * avoid race conditions with mmu_notifier that might try to free them.
+ *
+ */
+void kfd_cleanup_processes(void)
+{
+	struct kfd_process *p;
+	struct hlist_node *p_temp;
+	unsigned int temp;
+	HLIST_HEAD(cleanup_list);
+
+	/*
+	 * Move all remaining kfd_process from the process table to a
+	 * temp list for processing.   Once done, callback from mmu_notifier
+	 * release will not see the kfd_process in the table and do early return,
+	 * avoiding double free issues.
+	 */
+	mutex_lock(&kfd_processes_mutex);
+	hash_for_each_safe(kfd_processes_table, temp, p_temp, p, kfd_processes) {
+		hash_del_rcu(&p->kfd_processes);
+		synchronize_srcu(&kfd_processes_srcu);
+		hlist_add_head(&p->kfd_processes, &cleanup_list);
+	}
+	mutex_unlock(&kfd_processes_mutex);
+
+	hlist_for_each_entry_safe(p, p_temp, &cleanup_list, kfd_processes)
+		kfd_process_notifier_release_internal(p);
+
+	/*
+	 * Ensures that all outstanding free_notifier get called, triggering
+	 * the release of the kfd_process struct.
+	 */
+	mmu_notifier_synchronize();
+}
+
 static int kfd_process_init_cwsr_apu(struct kfd_process *p, struct file *filep)
 {
 	unsigned long  offset;