drm/amdkfd: Make kfd_process reference counted

This will be used to elliminate the use of the process lock for
preventing concurrent process destruction. This will simplify lock
dependencies between KFD and KGD.

This also simplifies the process destruction in a few ways:
* Don't allocate work struct dynamically
* Remove unnecessary hack that increments mm reference counter
* Remove unnecessary process locking during destruction
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Oded Gabbay <oded.gabbay@gmail.com>
Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>

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

@@ -34,6 +34,7 @@
 #include <linux/idr.h>
 #include <linux/kfifo.h>
 #include <linux/seq_file.h>
+#include <linux/kref.h>
 #include <kgd_kfd_interface.h>
 
 #include "amd_shared.h"
@@ -537,6 +538,9 @@ struct kfd_process {
 	 */
 	void *mm;
 
+	struct kref ref;
+	struct work_struct release_work;
+
 	struct mutex mutex;
 
 	/*

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

@@ -48,11 +48,6 @@ DEFINE_STATIC_SRCU(kfd_processes_srcu);
 
 static struct workqueue_struct *kfd_process_wq;
 
-struct kfd_process_release_work {
-	struct work_struct kfd_work;
-	struct kfd_process *p;
-};
-
 static struct kfd_process *find_process(const struct task_struct *thread);
 static struct kfd_process *create_process(const struct task_struct *thread);
 static int kfd_process_init_cwsr(struct kfd_process *p, struct file *filep);
@@ -151,21 +146,20 @@ static struct kfd_process *find_process(const struct task_struct *thread)
 	return p;
 }
 
+/* No process locking is needed in this function, because the process
+ * is not findable any more. We must assume that no other thread is
+ * using it any more, otherwise we couldn't safely free the process
+ * structure in the end.
+ */
 static void kfd_process_wq_release(struct work_struct *work)
 {
-	struct kfd_process_release_work *my_work;
+	struct kfd_process *p = container_of(work, struct kfd_process,
+					     release_work);
 	struct kfd_process_device *pdd, *temp;
-	struct kfd_process *p;
-
-	my_work = (struct kfd_process_release_work *) work;
-
-	p = my_work->p;
 
 	pr_debug("Releasing process (pasid %d) in workqueue\n",
 			p->pasid);
 
-	mutex_lock(&p->mutex);
-
 	list_for_each_entry_safe(pdd, temp, &p->per_device_data,
 							per_device_list) {
 		pr_debug("Releasing pdd (topology id %d) for process (pasid %d) in workqueue\n",
@@ -188,33 +182,26 @@ static void kfd_process_wq_release(struct work_struct *work)
 	kfd_pasid_free(p->pasid);
 	kfd_free_process_doorbells(p);
 
-	mutex_unlock(&p->mutex);
-
 	mutex_destroy(&p->mutex);
 
 	put_task_struct(p->lead_thread);
 
 	kfree(p);
-
-	kfree(work);
 }
 
-static void kfd_process_destroy_delayed(struct rcu_head *rcu)
+static void kfd_process_ref_release(struct kref *ref)
 {
-	struct kfd_process_release_work *work;
-	struct kfd_process *p;
-
-	p = container_of(rcu, struct kfd_process, rcu);
+	struct kfd_process *p = container_of(ref, struct kfd_process, ref);
 
-	mmdrop(p->mm);
+	INIT_WORK(&p->release_work, kfd_process_wq_release);
+	queue_work(kfd_process_wq, &p->release_work);
+}
 
-	work = kmalloc(sizeof(struct kfd_process_release_work), GFP_ATOMIC);
+static void kfd_process_destroy_delayed(struct rcu_head *rcu)
+{
+	struct kfd_process *p = container_of(rcu, struct kfd_process, rcu);
 
-	if (work) {
-		INIT_WORK((struct work_struct *) work, kfd_process_wq_release);
-		work->p = p;
-		queue_work(kfd_process_wq, (struct work_struct *) work);
-	}
+	kref_put(&p->ref, kfd_process_ref_release);
 }
 
 static void kfd_process_notifier_release(struct mmu_notifier *mn,
@@ -258,15 +245,12 @@ static void kfd_process_notifier_release(struct mmu_notifier *mn,
 	kfd_process_dequeue_from_all_devices(p);
 	pqm_uninit(&p->pqm);
 
+	/* Indicate to other users that MM is no longer valid */
+	p->mm = NULL;
+
 	mutex_unlock(&p->mutex);
 
-	/*
-	 * Because we drop mm_count inside kfd_process_destroy_delayed
-	 * and because the mmu_notifier_unregister function also drop
-	 * mm_count we need to take an extra count here.
-	 */
-	mmgrab(p->mm);
-	mmu_notifier_unregister_no_release(&p->mmu_notifier, p->mm);
+	mmu_notifier_unregister_no_release(&p->mmu_notifier, mm);
 	mmu_notifier_call_srcu(&p->rcu, &kfd_process_destroy_delayed);
 }
 
@@ -331,6 +315,8 @@ static struct kfd_process *create_process(const struct task_struct *thread)
 	if (kfd_alloc_process_doorbells(process) < 0)
 		goto err_alloc_doorbells;
 
+	kref_init(&process->ref);
+
 	mutex_init(&process->mutex);
 
 	process->mm = thread->mm;