Date: Thu, 11 Apr 2019 20:51:20 +0300 From: Konstantin Belousov <kostikbel@gmail.com> To: Conrad Meyer <cem@freebsd.org> Cc: Edward Tomasz Napierala <trasz@freebsd.org>, src-committers <src-committers@freebsd.org>, svn-src-all <svn-src-all@freebsd.org>, svn-src-head <svn-src-head@freebsd.org> Subject: Re: svn commit: r346120 - head/sys/kern Message-ID: <20190411175120.GX1923@kib.kiev.ua> In-Reply-To: <CAG6CVpXo%2BBwML8z%2BsNZ-d2cHLoym0nAeZ3WP60ugZVn85FqT-A@mail.gmail.com> References: <201904111121.x3BBLj2K023087@repo.freebsd.org> <CAG6CVpXo%2BBwML8z%2BsNZ-d2cHLoym0nAeZ3WP60ugZVn85FqT-A@mail.gmail.com>
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On Thu, Apr 11, 2019 at 08:59:14AM -0700, Conrad Meyer wrote: > Hi Edward, > > I have a question about this change below. > > On Thu, Apr 11, 2019 at 4:22 AM Edward Tomasz Napierala > <trasz@freebsd.org> wrote: > > > > Author: trasz > > Date: Thu Apr 11 11:21:45 2019 > > New Revision: 346120 > > URL: https://svnweb.freebsd.org/changeset/base/346120 > > > > Log: > > Use shared vnode locks for the ELF interpreter. > > > > ... > > Differential Revision: https://reviews.freebsd.org/D19874 > > ... > > Modified: head/sys/kern/imgact_elf.c > > ============================================================================== > > --- head/sys/kern/imgact_elf.c Thu Apr 11 08:06:45 2019 (r346119) > > +++ head/sys/kern/imgact_elf.c Thu Apr 11 11:21:45 2019 (r346120) > > ... > > - NDINIT(nd, LOOKUP, LOCKLEAF | FOLLOW, UIO_SYSSPACE, file, curthread); > > + flags = FOLLOW | LOCKSHARED | LOCKLEAF; > > + > > +again: > > + NDINIT(nd, LOOKUP, flags, UIO_SYSSPACE, file, curthread); > > if ((error = namei(nd)) != 0) { > > ... > > @@ -759,15 +762,30 @@ __elfN(load_file)(struct proc *p, const char *file, u_ > > ... > > + if (VOP_IS_TEXT(nd->ni_vp) == 0) { > > + if (VOP_ISLOCKED(nd->ni_vp) != LK_EXCLUSIVE) { > > + /* > > + * LK_UPGRADE could have resulted in dropping > > + * the lock. Just try again from the start, > > + * this time with exclusive vnode lock. > > + */ > > + vput(nd->ni_vp); > > + flags &= ~LOCKSHARED; > > + goto again; > > It's unclear to me why we don't attempt LK_UPGRADE first. If upgrade > succeeds, we avoid an extra filesystem traversal (namei/lookup). If > it fails, of course we can 'goto again' the same as we do > unconditionally here. LK_UPGRADE drops the vnode lock, this was discussed in the phab review. Perhaps you can do LK_TRYUPGRADE first, by the cost of even more complicated code then was in the initial review, if fixed. > > There was some discussion about the topic in the linked phabricator PR > with Konstantin, but I did not follow it fully. > > On the one hand, perhaps VOP_IS_TEXT() is rarely false for common > interpreters anyway. On the other hand, there is sort of a > renaissance of static linking happening. Statically linked binaries do not need interpreter. > So maybe the thought is, > !VOP_IS_TEXT is likely to be rare, and LK_UPGRADE success even more > rare, so why bother writing additional code for it? I do not understand the question. > > Thanks, > Conrad > > P.S., It is orthogonal to this discussion, but I don't see any reason > for VOP_IS_TEXT to be a vnode_if operation. Neither it, nor > VOP_UNSET_TEXT, is ever specialized. They simply check or clear the > VV_TEXT flag on the vnode's vflags, respectively. It is common for > the kernel to reach out and interact with other vnode vflags directly; > e.g., pretty much all other VV_flags, like VV_ROOT. The only > specialization of VOP_SET_TEXT is NFSclient, and it is unclear to me > why the same requirements NFS client has for setting VV_TEXT do not > apply universally. VOP is bypassed, unlike direct vnode flags access. As result, when you execute file on nullfs, VV_TEXT is set on the lower vnode, instead of the upper nullfs vnode. If lower vnode is not marked by VV_TEXT, EBUSY does not work. This was described in the commit log where VOPs were introduced.
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