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


Regards,
Luis
-- 
[ Luis Claudio R. Goncalves                    Bass - Gospel - RT ]
[ Fingerprint: 4FDD B8C4 3C59 34BD 8BE9  2696 7203 D980 A448 C8F8 ]

--

[ message continues ]

I don't have the -rt tree at hand; can you copy a few lines of code
around line 648 of fs/namespace.c, please?

--

[ message continues ]

On Wed, May 05, 2010 at 12:27:56AM +1000, Nick Piggin wrote:
| On Tue, May 04, 2010 at 11:04:44AM -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()
| 
| I don't have the -rt tree at hand; can you copy a few lines of code
| around line 648 of fs/namespace.c, please?

Line 648 is the "WARN_ON(mnt->mnt_flags & MNT_MOUNTED);" in this function:


/*
 * the caller must hold vfsmount_lock
 */
static void commit_tree(struct vfsmount *mnt)
{
        struct vfsmount *parent = mnt->mnt_parent;
        struct vfsmount *m;
        LIST_HEAD(head);
        struct mnt_namespace *n = parent->mnt_ns;

        BUG_ON(parent == mnt);

        list_add_tail(&head, &mnt->mnt_list);
        list_for_each_entry(m, &head, mnt_list)
                m->mnt_ns = n;
        list_splice(&head, n->list.prev);

        list_add_tail(&mnt->mnt_hash, mount_hashtable +
                                hash(parent, mnt->mnt_mountpoint));
        list_add_tail(&mnt->mnt_child, &parent->mnt_mounts);
        WARN_ON(mnt->mnt_flags & MNT_MOUNTED);
        mnt->mnt_flags |= MNT_MOUNTED;
        touch_mnt_namespace(n);
}


Luis
-- 
[ Luis Claudio R. Goncalves                    Bass - Gospel - RT ]
[ Fingerprint: 4FDD B8C4 3C59 34BD 8BE9  2696 7203 D980 A448 C8F8 ]

--

[ message continues ]

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 ...
[ message continues ]

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.


--

[ message continues ]