Date:	Wed, 5 May 2010 18:08:34 +1000
From:	Nick Piggin <npiggin@...e.de>
To:	john stultz <johnstul@...ibm.com>
Cc:	"Luis Claudio R. Goncalves" <lclaudio@...g.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	rt-users <linux-rt-users@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RT] 2.6.33.3-rt19: WARNING: at fs/namespace.c:648
 commit_tree+0xf1/0x10b()

On Tue, May 04, 2010 at 06:14:19PM -0700, John Stultz wrote:
> On Tue, 2010-05-04 at 11:04 -0300, Luis Claudio R. Goncalves wrote:
> > John,
> > 
> > As the backtrace seems to be closely related to what has been discussed on
> > the thread "2.6.33.3-rt16: WARNING: at fs/namespace.c:1197", I copied the
> > same people on this message.
> > 
> > As a side note, this time I just see the warning, there is no system freeze
> > involved.
> > 
> > 
> > ------------[ cut here ]------------
> > WARNING: at fs/namespace.c:648 commit_tree+0xf1/0x10b()
> > Hardware name: KQ260AA-AC4 a6540br
> > Modules linked in: nls_utf8 udf vfat fat usb_storage fuse i915 drm_kms_helper drm video output ipt_MASQUERADE iptable_nat nf_nat bridge stp llc sunrpc ipv6 xt_physdev ipt_REJECT xt_tcpudp nf_conntrack_ipv4 nf_defrag_ipv4 xt_state iptable_filter ip_tables x_tables dm_mirror dm_region_hash dm_log dm_multipath scsi_dh dm_mod kvm_intel kvm uinput tuner_simple tuner_types wm8775 tda9887 tda8290 snd_hda_codec_realtek tuner snd_hda_intel cx25840 snd_hda_codec ivtv snd_hwdep snd_seq i2c_algo_bit cx2341x v4l2_common videodev snd_seq_device v4l1_compat snd_pcm v4l2_compat_ioctl32 snd_timer ir_common snd ir_core sg r8169 tveeprom soundcore sr_mod i2c_i801 mii iTCO_wdt snd_page_alloc intel_agp firewire_ohci serio_raw cdrom i2c_core iTCO_vendor_support firewire_core pcspkr crc_itu_t button ahci libata sd_mod scsi_mod crc_t10dif ext3 jbd mbcache uhci_hcd ohci_hcd ehci_hcd [last unloaded: microcode]
> > Pid: 14002, comm: fusermount Not tainted 2.6.33.3-rt19 #32
> > Call Trace:
> > [<ffffffff81040e67>] warn_slowpath_common+0x7c/0x94
> > [<ffffffff81040e93>] warn_slowpath_null+0x14/0x16
> > [<ffffffff81115811>] commit_tree+0xf1/0x10b
> > [<ffffffff8111661d>] attach_recursive_mnt+0xf2/0x188
> > [<ffffffff811167b3>] graft_tree+0x100/0x102
> > [<ffffffff8111765b>] do_mount+0x386/0x7ae
> > [<ffffffff810d55f2>] ? strndup_user+0x5d/0x85
> > [<ffffffff81117b0b>] sys_mount+0x88/0xc2
> > [<ffffffff81002d32>] system_call_fastpath+0x16/0x1b
> 
> Yea. Looks like the fuse mounts are more interesting here and are
> tripping up the MNT_MOUNTED logic.
> 
> I have a patch that will likely resolve this, but I don't think its
> right, because all the MNT_MOUNTED corner cases are starting to pile up
> and I suspect a deeper fix is needed.
> 
> 
> Nick, maybe you can help here?
> 
> Trivial cases:
> MNT_MOUNTED gets set by:
> 	attach_mnt
> 	commit_tree
> 
> MNT_MOUNTED gets unset by:
> 	detach_mnt
> 	unmount_tree
> 
> So there's a nice symmetry there.
> 
> We also clear MNT_MOUNTED in clone_mnt(), since we're creating a
> unmounted copy that we will latter call attach_mnt() upon.
> 
> 
> Now, here's where things get messy:
> copy_tree():
> 	In your patches, we didn't set MNT_MOUNTED on the first clone on the
> root of the mnt to be copied. This caused problems with new namespaces
> since after it is copied, we don't call attach_mnt or commit_tree. So
> when the namespace is removed, and we call unmount_tree, and hit a
> WARN_ON. Similarly, if we bombed out in copy_tree due to a ENOMEM, we
> call umount_tree on the mnt and will hit the WARN_ON as well. The same
> issue hits us with collect_mounts and drop_collected_mounts, where we
> copy_tree() and then unmount_tree() and hit the WARN_ON.
> 
> This seemed broken, so I set MNT_MOUNTED on the root cloned mnt in
> copy_tree and it resolved the above asymmetries. 
> 
> However, do_loopback is more complicated, since it calls either
> copy_tree or clone_mnt  (depending on the recursive flag) and then
> grafts that mnt which calls commit_tree()/attach_mnt(). 
> 
> Leaving clone_mnt(), the mnt is not set as MNT_MOUNTED, but now with my
> change to copy_tree(), it sets the root as MNT_MOUNTED. This then causes
> a WARN_ON in the commit_tree() called by graft_tree().
> 
> The hacky fix below simply clears the recently set MNT_MOUNTED flag
> after copy_tree() returns,  before calling graft_tree().
> 
> Now, I'm not very clear on the mnt rules here, so I'm probably wrong.
> And it just seems so hacky there probably should be a better fix.
> 
> Ideas:
> o Maybe the callers of copy_tree() should be setting the MNT_MOUNTED
> flag? (and the ENOMEM case there would still need to be cleaned up). 
> 
> o Should we only call unmount_tree on the first sub mnt, instead of on
> the non MNT_MOUNTED mnt?
> 
> o Or maybe the WARN_ONs are just overly cautious, and we should be cool
> calling unmount on mnts not marked MNT_MOUNTED?
> 
> Do you have any thoughts here? 

I guess keeping MNT_MOUNTED in sync with !list_empty(&mnt->mnt_hash)
should work. I think it would just need fixing up there.

I'm increasingly of the idea that MNT_MOUNTED is not such a good idea,
though. I don't know how common it is to run with detached mount point
(eg with a lazy umount), but in that case it would go much slower, which
isn't nice.

So I was looking at other ways to do scalable refcounting. It's tricky
though. I'm thinking either account other long-lived refcounts similarly
to MNT_MOUNTED (obviously needs to be a counter rather than a flag
then), such as fs->root and fs->cwd; or using a lazy scheme which just
periodically checks for 0 refcount. Either is going to be a bit tricky.


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