drm/amdkfd: svm range restore work deadlock when process exit

kfd_process_notifier_release flush svm_range_restore_work
which calls svm_range_list_lock_and_flush_work to flush deferred_list
work, but if deferred_list work mmput release the last user, it will
call exit_mmap -> notifier_release, it is deadlock with below backtrace.

Move flush svm_range_restore_work to kfd_process_wq_release to avoid
deadlock. Then svm_range_restore_work take task->mm ref to avoid mm is
gone while validating and mapping ranges to GPU.

Workqueue: events svm_range_deferred_list_work [amdgpu]
Call Trace:
 wait_for_completion+0x94/0x100
 __flush_work+0x12a/0x1e0
 __cancel_work_timer+0x10e/0x190
 cancel_delayed_work_sync+0x13/0x20
 kfd_process_notifier_release+0x98/0x2a0 [amdgpu]
 __mmu_notifier_release+0x74/0x1f0
 exit_mmap+0x170/0x200
 mmput+0x5d/0x130
 svm_range_deferred_list_work+0x104/0x230 [amdgpu]
 process_one_work+0x220/0x3c0
Signed-off-by: Philip Yang <Philip.Yang@amd.com>
Reported-by: Ruili Ji <ruili.ji@amd.com>
Tested-by: Ruili Ji <ruili.ji@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_process.c

@@ -1150,7 +1150,6 @@ static void kfd_process_notifier_release(struct mmu_notifier *mn,
 
 	cancel_delayed_work_sync(&p->eviction_work);
 	cancel_delayed_work_sync(&p->restore_work);
-	cancel_delayed_work_sync(&p->svms.restore_work);
 
 	mutex_lock(&p->mutex);
 

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

@@ -1643,13 +1643,14 @@ static void svm_range_restore_work(struct work_struct *work)
 
 	pr_debug("restore svm ranges\n");
 
-	/* kfd_process_notifier_release destroys this worker thread. So during
-	 * the lifetime of this thread, kfd_process and mm will be valid.
-	 */
 	p = container_of(svms, struct kfd_process, svms);
-	mm = p->mm;
-	if (!mm)
+
+	/* Keep mm reference when svm_range_validate_and_map ranges */
+	mm = get_task_mm(p->lead_thread);
+	if (!mm) {
+		pr_debug("svms 0x%p process mm gone\n", svms);
 		return;
+	}
 
 	svm_range_list_lock_and_flush_work(svms, mm);
 	mutex_lock(&svms->lock);
@@ -1703,6 +1704,7 @@ static void svm_range_restore_work(struct work_struct *work)
 out_reschedule:
 	mutex_unlock(&svms->lock);
 	mmap_write_unlock(mm);
+	mmput(mm);
 
 	/* If validation failed, reschedule another attempt */
 	if (evicted_ranges) {
@@ -2840,6 +2842,8 @@ void svm_range_list_fini(struct kfd_process *p)
 
 	pr_debug("pasid 0x%x svms 0x%p\n", p->pasid, &p->svms);
 
+	cancel_delayed_work_sync(&p->svms.restore_work);
+
 	/* Ensure list work is finished before process is destroyed */
 	flush_work(&p->svms.deferred_list_work);
 
@@ -2850,7 +2854,6 @@ void svm_range_list_fini(struct kfd_process *p)
 	atomic_inc(&p->svms.drain_pagefaults);
 	svm_range_drain_retry_fault(&p->svms);
 
-
 	list_for_each_entry_safe(prange, next, &p->svms.list, list) {
 		svm_range_unlink(prange);
 		svm_range_remove_notifier(prange);