svn commit: r334708 - head/sys/kern

[Justin Hibbits]

Author: jhibbits
Date: Wed Jun  6 12:57:11 2018
New Revision: 334708
URL: https://svnweb.freebsd.org/changeset/base/334708

Log:
  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.

Modified:
  head/sys/kern/vfs_subr.c

Modified: head/sys/kern/vfs_subr.c
==============================================================================
--- head/sys/kern/vfs_subr.c	Wed Jun  6 10:46:24 2018	(r334707)
+++ head/sys/kern/vfs_subr.c	Wed Jun  6 12:57:11 2018	(r334708)
@@ -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;