From 0ec9835421b7844e9244a78ce3d27bf21984963e Mon Sep 17 00:00:00 2001
From: cperciva <cperciva@ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f>
Date: Sat, 27 Feb 2010 10:55:43 +0000
Subject: [PATCH] MFC r197223: Fix a deadlock in the ULE scheduler.

Approved by:	so (cperciva)
Errata:		FreeBSD-EN-10:02.sched_ule


git-svn-id: svn://svn.freebsd.org/base/releng/7.2@204409 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f
---
 UPDATING             |  3 +++
 sys/conf/newvers.sh  |  2 +-
 sys/kern/sched_ule.c | 24 +++++++++++++++---------
 3 files changed, 19 insertions(+), 10 deletions(-)

diff --git a/UPDATING b/UPDATING
index 4bbe4308..f6e94bf8 100644
--- a/UPDATING
+++ b/UPDATING
@@ -8,6 +8,9 @@ Items affecting the ports and packages system can be found in
 /usr/ports/UPDATING.  Please read that file before running
 portupgrade.
 
+20100227:	p7	FreeBSD-EN-10:02.sched_ule
+	Fix a deadlock in the ULE scheduler.
+
 20100106:	p6	FreeBSD-SA-10:01.bind, FreeBSD-SA-10:02.ntpd,
 			FreeBSD-SA-10:03.zfs
 	Fix BIND named(8) cache poisoning with DNSSEC validation.
diff --git a/sys/conf/newvers.sh b/sys/conf/newvers.sh
index 87371582..80aa476f 100644
--- a/sys/conf/newvers.sh
+++ b/sys/conf/newvers.sh
@@ -32,7 +32,7 @@
 
 TYPE="FreeBSD"
 REVISION="7.2"
-BRANCH="RELEASE-p6"
+BRANCH="RELEASE-p7"
 if [ "X${BRANCH_OVERRIDE}" != "X" ]; then
 	BRANCH=${BRANCH_OVERRIDE}
 fi
diff --git a/sys/kern/sched_ule.c b/sys/kern/sched_ule.c
index a9d4e447..0340a35c 100644
--- a/sys/kern/sched_ule.c
+++ b/sys/kern/sched_ule.c
@@ -1822,18 +1822,24 @@ 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(td->td_sched);
+
 	/*
-	 * 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);
+	if (TDQ_LOCKPTR(tdn) == TDQ_LOCKPTR(tdq)) {
 		TDQ_LOCK(tdq);
+		tdq_add(tdn, td, flags);
+		tdq_notify(td->td_sched);
+	} else {
+		tdq_lock_pair(tdn, tdq);
+		tdq_add(tdn, td, flags);
+		tdq_notify(td->td_sched);
+		TDQ_UNLOCK(tdn);
 	}
 	spinlock_exit();
 #endif
-- 
2.45.0