Date: Wed, 6 Jun 2018 16:22:51 +0300 From: Andriy Gapon <avg@FreeBSD.org> To: Warner Losh <imp@bsdimp.com>, Justin Hibbits <jhibbits@freebsd.org> Cc: src-committers <src-committers@freebsd.org>, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: Re: svn commit: r334708 - head/sys/kern Message-ID: <600c9c66-fda6-de4c-3db7-0a79ef833f62@FreeBSD.org> In-Reply-To: <CANCZdfq0E9u-_GiSdBrZWsH53pV%2Bp_cp1FESRzfDzepDZgkTcQ@mail.gmail.com> References: <201806061257.w56CvCwq089369@repo.freebsd.org> <CANCZdfq0E9u-_GiSdBrZWsH53pV%2Bp_cp1FESRzfDzepDZgkTcQ@mail.gmail.com>
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On 06/06/2018 16:02, Warner Losh wrote: > > > On Wed, Jun 6, 2018 at 8:57 AM, Justin Hibbits <jhibbits@freebsd.org > <mailto:jhibbits@freebsd.org>> wrote: > > Author: jhibbits > Date: Wed Jun 6 12:57:11 2018 > New Revision: 334708 > URL: https://svnweb.freebsd.org/changeset/base/334708 > <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; > > > So why isn't the refcount_acquire() enough? This is the wrong "acquire", "acquire" in a sense of "get" or "increment", not in a sense of a memory barrier. -- Andriy Gapon
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