From b3460f44524b145c6c8a760ebe65052560a810bf Mon Sep 17 00:00:00 2001
From: Wei Hu <whu@FreeBSD.org>
Date: Thu, 15 Oct 2020 11:44:28 +0000
Subject: [PATCH] Hyper-V: hn: Relinquish cpu in HN_LOCK to avoid deadlock

The try lock loop in HN_LOCK put the thread spinning on cpu if the lock
is not available. It is possible to cause deadlock if the thread holding
the lock is sleeping. Relinquish the cpu to work around this problem even
it doesn't completely solve the issue. The priority inversion could cause
the livelock no matter how less likely it could happen. A more complete
solution may be needed in the future.

Reported by:	Microsoft, Netapp
MFC after:	2 weeks
Sponsored by:	Microsoft
---
 sys/dev/hyperv/netvsc/if_hn.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/sys/dev/hyperv/netvsc/if_hn.c b/sys/dev/hyperv/netvsc/if_hn.c
index 6c1e62c22d6f..5a57ea796d0e 100644
--- a/sys/dev/hyperv/netvsc/if_hn.c
+++ b/sys/dev/hyperv/netvsc/if_hn.c
@@ -71,8 +71,10 @@ __FBSDID("$FreeBSD$");
 #include <sys/module.h>
 #include <sys/queue.h>
 #include <sys/lock.h>
+#include <sys/proc.h>
 #include <sys/rmlock.h>
 #include <sys/sbuf.h>
+#include <sys/sched.h>
 #include <sys/smp.h>
 #include <sys/socket.h>
 #include <sys/sockio.h>
@@ -165,8 +167,11 @@ __FBSDID("$FreeBSD$");
 #define HN_LOCK_ASSERT(sc)		sx_assert(&(sc)->hn_lock, SA_XLOCKED)
 #define HN_LOCK(sc)					\
 do {							\
-	while (sx_try_xlock(&(sc)->hn_lock) == 0)	\
+	while (sx_try_xlock(&(sc)->hn_lock) == 0) {	\
+		/* Relinquish cpu to avoid deadlock */	\
+		sched_relinquish(curthread);		\
 		DELAY(1000);				\
+	}						\
 } while (0)
 #define HN_UNLOCK(sc)			sx_xunlock(&(sc)->hn_lock)