Date: Wed, 24 Sep 2014 17:34:14 +0200 From: Peter Holm <peter@holm.cc> To: Bryan Drewery <bdrewery@freebsd.org> Cc: FreeBSD FS <freebsd-fs@freebsd.org> Subject: Re: Deadlock with umount -f involving tmpfs on top of ZFS on r271170 Message-ID: <20140924153414.GA15739@x2.osted.lan> In-Reply-To: <5422E15F.8050702@FreeBSD.org> References: <5420D5FC.4030600@FreeBSD.org> <20140923131244.GC8870@kib.kiev.ua> <5422240F.4080003@FreeBSD.org> <20140924102758.GH8870@kib.kiev.ua> <20140924132605.GA11772@x2.osted.lan> <5422E15F.8050702@FreeBSD.org>
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On Wed, Sep 24, 2014 at 10:21:03AM -0500, Bryan Drewery wrote: > On 9/24/2014 8:26 AM, Peter Holm wrote: > > On Wed, Sep 24, 2014 at 01:27:58PM +0300, Konstantin Belousov wrote: > >> On Tue, Sep 23, 2014 at 08:53:19PM -0500, Bryan Drewery wrote: > >>> I tried your patch and still ran into the deadlock. Here is the debug > >>> information: https://people.freebsd.org/~bdrewery/vfs_deadlock.2.txt > >> > >> I see what is going on. Previous patch cannot help there. > >> > >> The problem is that kern_linkat() starts write with vn_start_write(9), > >> and then tries to execute namei(9) with a path potentially crossing > >> the mount point. If the mount point is being unmounted, unmount cannot > >> lay suspension on the filesystem due to writer, but it owns the covered > >> vnode lock. On the other hand, namei(9) is unable to cross the mount > >> point since covered vnode is locked, thus writer does not make progress. > >> > >> The similar issue exists in rename code. The deadlock is only possible > >> for filesystems which suspend writes on unmount; such fs are UFS and tmpfs. > >> > >> Try this. > >> > >> diff --git a/sys/kern/vfs_syscalls.c b/sys/kern/vfs_syscalls.c > >> index b3b7ed5..9775480 100644 > > > > I had been trying different scenarios without much luck, but with > > your description I got the problem instantly. :) > > > > The patch is an improvement, but: > > > > http://people.freebsd.org/~pho/stress/log/kostik718.txt > > > > - Peter > > > > By the way this was my test script: > > http://dpaste.com/35AYZZ7 > > It could be simplified a lot. > Thank you. Here's what I came up with: http://people.freebsd.org/~pho/link.sh - Peter
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