1 files changed, 11 insertions, 11 deletions

diff --git a/sys/kern/sched_ule.c b/sys/kern/sched_ule.c
index 27e8371dff2b..fd14fc4021a3 100644
--- a/sys/kern/sched_ule.c
+++ b/sys/kern/sched_ule.c
@@ -1749,19 +1749,19 @@ sched_switch_migrate(struct tdq *tdq, struct thread *td, int flags)
 	 */
 	spinlock_enter();
 	thread_block_switch(td);	/* This releases the lock on tdq. */
-	TDQ_LOCK(tdn);
-	tdq_add(tdn, td, flags);
-	tdq_notify(tdn, td);
+
 	/*
-	 * After we unlock tdn the new cpu still can't switch into this
-	 * thread until we've unblocked it in cpu_switch().  The lock
-	 * pointers may match in the case of HTT cores.  Don't unlock here
-	 * or we can deadlock when the other CPU runs the IPI handler.
+	 * Acquire both run-queue locks before placing the thread on the new
+	 * run-queue to avoid deadlocks created by placing a thread with a
+	 * blocked lock on the run-queue of a remote processor.  The deadlock
+	 * occurs when a third processor attempts to lock the two queues in
+	 * question while the target processor is spinning with its own
+	 * run-queue lock held while waiting for the blocked lock to clear.
 	 */
-	if (TDQ_LOCKPTR(tdn) != TDQ_LOCKPTR(tdq)) {
-		TDQ_UNLOCK(tdn);
-		TDQ_LOCK(tdq);
-	}
+	tdq_lock_pair(tdn, tdq);
+	tdq_add(tdn, td, flags);
+	tdq_notify(tdn, td);
+	TDQ_UNLOCK(tdn);
 	spinlock_exit();
 #endif
 	return (TDQ_LOCKPTR(tdn));