From 32c369f40c3f123140e08d6f60b1ac6603a007e6 Mon Sep 17 00:00:00 2001
From: Justin Hibbits <jhibbits@FreeBSD.org>
Date: Wed, 6 Jun 2018 12:57:11 +0000
Subject: [PATCH] Add a memory barrier after taking a reference on the vnode
holdcnt in _vhold
This is needed to avoid a race between the VNASSERT() below, and another
thread updating the VI_FREE flag, on weakly-ordered architectures.
On a 72-thread POWER9, without this barrier a 'make -j72 buildworld' would
panic on the assert regularly.
It may be possible to use a weaker barrier, and I'll investigate that once
all stability issues are worked out on POWER9.
---
sys/kern/vfs_subr.c | 3 +++
1 file changed, 3 insertions(+)
diff --git a/sys/kern/vfs_subr.c b/sys/kern/vfs_subr.c
index 055d3a828bf4..286a871c3631 100644
--- a/sys/kern/vfs_subr.c
+++ b/sys/kern/vfs_subr.c
@@ -2807,6 +2807,9 @@ _vhold(struct vnode *vp, bool locked)
CTR2(KTR_VFS, "%s: vp %p", __func__, vp);
if (!locked) {
if (refcount_acquire_if_not_zero(&vp->v_holdcnt)) {
+#if !defined(__amd64__) && !defined(__i386__)
+ mb();
+#endif
VNASSERT((vp->v_iflag & VI_FREE) == 0, vp,
("_vhold: vnode with holdcnt is free"));
return;