new-bus: Add taskqueue_bus to process hot-plug device events

Use a system-wide taskqueue for hot-plug events.  This avoids possibly
blocking unrelated events on the thread taskqueue without requiring
multiple driver-specific taskqueues.

Reviewed by:	imp
Differential Revision:	https://reviews.freebsd.org/D49268

(cherry picked from commit 44d5f5ed1e)

sys/kern/subr_bus.c

@@ -51,6 +51,7 @@
 #include <sys/smp.h>
 #include <sys/sysctl.h>
 #include <sys/systm.h>
+#include <sys/taskqueue.h>
 #include <sys/bus.h>
 #include <sys/cpuset.h>
 #ifdef INTRNG
@@ -120,6 +121,8 @@ struct device_prop_elm {
 	LIST_ENTRY(device_prop_elm) link;
 };
 
+TASKQUEUE_DEFINE_THREAD(bus);
+
 static void device_destroy_props(device_t dev);
 
 /**

sys/sys/taskqueue.h

@@ -215,4 +215,10 @@ struct taskqueue *taskqueue_create_fast(const char *name, int mflags,
 				    taskqueue_enqueue_fn enqueue,
 				    void *context);
 
+/*
+ * This queue is used to process asynchronous device events such as
+ * hot plug insertion and removal of devices.
+ */
+TASKQUEUE_DECLARE(bus);
+
 #endif /* !_SYS_TASKQUEUE_H_ */