drm/amdkfd: Flush the process wq before creating a kfd_process

There is a race condition when re-creating a kfd_process for a process.
This has been observed when a process under the debugger executes
exec(3). In this scenario:
- The process executes exec.
- This will eventually release the process's mm, which will cause the
kfd_process object associated with the process to be freed
(kfd_process_free_notifier decrements the reference count to the
kfd_process to 0). This causes kfd_process_ref_release to enqueue
kfd_process_wq_release to the kfd_process_wq.
- The debugger receives the PTRACE_EVENT_EXEC notification, and tries to
re-enable AMDGPU traps (KFD_IOC_DBG_TRAP_ENABLE).
- When handling this request, KFD tries to re-create a kfd_process.
This eventually calls kfd_create_process and kobject_init_and_add.

At this point the call to kobject_init_and_add can fail because the
old kfd_process.kobj has not been freed yet by kfd_process_wq_release.

This patch proposes to avoid this race by making sure to drain
kfd_process_wq before creating a new kfd_process object. This way, we
know that any cleanup task is done executing when we reach
kobject_init_and_add.

Signed-off-by: Lancelot SIX <[e-mail]>
Reviewed-by: Felix Kuehling <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>

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

···
 	if (process) {
 		pr_debug("Process already found\n");
 	} else {
+		/* If the process just called exec(3), it is possible that the
+		 * cleanup of the kfd_process (following the release of the mm
+		 * of the old process image) is still in the cleanup work queue.
+		 * Make sure to drain any job before trying to recreate any
+		 * resource for this process.
+		 */
+		flush_workqueue(kfd_process_wq);
+
 		process = create_process(thread);
 		if (IS_ERR(process))
 			goto out;