Date: Wed, 6 Jun 2018 12:57:12 +0000 (UTC) From: Justin Hibbits <jhibbits@FreeBSD.org> To: src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: svn commit: r334708 - head/sys/kern Message-ID: <201806061257.w56CvCwq089369@repo.freebsd.org>
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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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