Initialize and free the DTrace taskqueue in the dtrace module load/unload

handlers rather than in the dtrace device open/close methods. The current
approach can cause a panic if the device is closed which the taskqueue
thread is active, or if a kernel module containing a provider is unloaded
while retained enablings are present and the dtrace device isn't opened.

Submitted by:	gibbs (original version)
Reviewed by:	gibbs
Approved by:	re (glebius)
MFC after:	2 weeks

sys/cddl/contrib/opensolaris/uts/common/dtrace/dtrace.c

@@ -15751,10 +15751,6 @@ dtrace_open(struct cdev *dev, int oflags, int devtype, struct thread *td)
 #else
 	devfs_set_cdevpriv(state, dtrace_dtr);
 #endif
-	/* This code actually belongs in dtrace_attach() */
-	if (dtrace_opens == 1)
-		dtrace_taskq = taskq_create("dtrace_taskq", 1, maxclsyspri,
-		    1, INT_MAX, 0);
 #endif
 
 	mutex_exit(&cpu_lock);
@@ -15842,11 +15838,6 @@ dtrace_dtr(void *data)
 		(void) kdi_dtrace_set(KDI_DTSET_DTRACE_DEACTIVATE);
 #else
 	--dtrace_opens;
-	/* This code actually belongs in dtrace_detach() */
-	if ((dtrace_opens == 0) && (dtrace_taskq != NULL)) {
-		taskq_destroy(dtrace_taskq);
-		dtrace_taskq = NULL;
-	}
 #endif
 
 	mutex_exit(&dtrace_lock);

sys/cddl/dev/dtrace/dtrace_load.c

@@ -56,6 +56,8 @@ dtrace_load(void *dummy)
 	/* Hang our hook for exceptions. */
 	dtrace_invop_init();
 
+	dtrace_taskq = taskq_create("dtrace_taskq", 1, maxclsyspri, 0, 0, 0);
+
 	/* Register callbacks for linker file load and unload events. */
 	dtrace_kld_load_tag = EVENTHANDLER_REGISTER(kld_load,
 	    dtrace_kld_load, NULL, EVENTHANDLER_PRI_ANY);

sys/cddl/dev/dtrace/dtrace_unload.c

@@ -127,6 +127,8 @@ dtrace_unload()
 	mutex_destroy(&dtrace_errlock);
 #endif
 
+	taskq_destroy(dtrace_taskq);
+
 	/* Reset our hook for exceptions. */
 	dtrace_invop_uninit();