drm/amdkfd: Check debug trap enable before write dbg_ev_file

In interrupt context, write dbg_ev_file will be run by work queue. It
will cause write dbg_ev_file execution after debug_trap_disable, which
will cause NULL pointer access.
v2: cancel work "debug_event_workarea" before set dbg_ev_file as NULL.
Signed-off-by: Lin.Cao <lincao12@amd.com>
Reviewed-by: Jonathan Kim <jonathan.kim@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -104,7 +104,8 @@ void debug_event_write_work_handler(struct work_struct *work)
 			struct kfd_process,
 			debug_event_workarea);
 
-	kernel_write(process->dbg_ev_file, &write_data, 1, &pos);
+	if (process->debug_trap_enabled && process->dbg_ev_file)
+		kernel_write(process->dbg_ev_file, &write_data, 1, &pos);
 }
 
 /* update process/device/queue exception status, write to descriptor
@@ -656,6 +657,7 @@ int kfd_dbg_trap_disable(struct kfd_process *target)
 	else if (target->runtime_info.runtime_state != DEBUG_RUNTIME_STATE_DISABLED)
 		target->runtime_info.runtime_state = DEBUG_RUNTIME_STATE_ENABLED;
 
+	cancel_work_sync(&target->debug_event_workarea);
 	fput(target->dbg_ev_file);
 	target->dbg_ev_file = NULL;