Date: Sun, 14 Oct 2012 15:12:40 +0300 From: Andriy Gapon <avg@FreeBSD.org> To: Pawel Jakub Dawidek <pjd@FreeBSD.org> Cc: "freebsd-fs@freebsd.org" <freebsd-fs@FreeBSD.org> Subject: Re: potential zfs/vfs trouble in force umount Message-ID: <507AAC38.3000709@FreeBSD.org> In-Reply-To: <20121014112546.GH1383@garage.freebsd.pl> References: <507A8954.3000702@FreeBSD.org> <20121014112546.GH1383@garage.freebsd.pl>
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on 14/10/2012 14:25 Pawel Jakub Dawidek said the following: > On Sun, Oct 14, 2012 at 12:43:48PM +0300, Andriy Gapon wrote: >> >> I think that there is the following potentially troublesome scenario. One >> thread does zil_commit and obtains a znode pointer using zfs_zget. At >> this point the thread doesn't have any locks on either the znode or its >> vnode. the only thing that is supposed to keep them around is a >> reference on the vnode. If a force umount is going on in parallel, the >> one of the first things it does is calling vflush(FORCECLOSE) (this >> happens before closing down zil). vflush force-reclaims all vnodes in >> this case (even when v_usecount > 0). So the znode in question gets >> destroyed. Later, when the first thread tries to dereference the znode >> pointer it would crash. > > The z_teardown_lock lock is held for reading for every VOP and zfs_umount() > obtains this lock for writing before calling vflush(FORCECLOSE) and sets > z_unmounted to true. This in turn will make every new VOP to return with > EIO. This ensures that no VOP is in-progress when vflush() is called. > What was/is not clear to me is whether zil operations are always called under z_teardown_lock (aka ZFS_ENTER)... -- Andriy Gapon
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