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;
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