new kernel warning when running xfstest ext4/271 on 3.12

[Eric Whitney <enwlinux@gmail.com>]

Since 3.12-rc1, I've noticed a new warning message in my kernel logs when
running ext4 regression tests with xfstests on both my x86-64 (KVM guest) and
ARM (Pandaboard ES) SUTs.  (I'm using xfstests-bld to run xfstests.)  It
remains visible in 3.12-rc4.

The warning appears when running ext4/271 after mounting the test file system
with the dioread_nolock option.

After running ext4/271, the kernel log contains the following trace repeated
21 times, apparently once for each write performed by dd in the test:

EXT4-fs (vdc): mounted filesystem without journal. Opts: dioread_nolock,noload
------------[ cut here ]------------
WARNING: CPU: 1 PID: 1366 at fs/ext4/page-io.c:205 ext4_put_io_end_defer+0xf5/0x100()
Modules linked in: snd_hda_intel snd_hda_codec snd_hwdep snd_pcm snd_timer psmouse serio_raw snd soundcore virtio_balloon snd_page_alloc i2c_piix4 mac_hid lp parport floppy
CPU: 1 PID: 1366 Comm: dd Not tainted 3.12.0-rc4-ext4testing+ #1
Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
 0000000000000009 ffff88003fd03d10 ffffffff816e3b66 0000000000000000
 ffff88003fd03d48 ffffffff8104eb4c ffff88003d1a1168 ffff88003d209c10
 0000000000000000 0000000000001000 ffff88003bb57490 ffff88003fd03d58
Call Trace:
 <IRQ>  [<ffffffff816e3b66>] dump_stack+0x45/0x56
 [<ffffffff8104eb4c>] warn_slowpath_common+0x8c/0xc0
 [<ffffffff8104eb9a>] warn_slowpath_null+0x1a/0x20
 [<ffffffff81245df5>] ext4_put_io_end_defer+0xf5/0x100
 [<ffffffff81245ec6>] ext4_end_bio+0xc6/0xe0
 [<ffffffff811d206d>] bio_endio+0x1d/0x30
 [<ffffffff8133313a>] blk_update_request+0xba/0x450
 [<ffffffff813333e0>] ? blk_update_request+0x360/0x450
 [<ffffffff813334f7>] blk_update_bidi_request+0x27/0xa0
 [<ffffffff81336720>] __blk_end_bidi_request+0x20/0x50
 [<ffffffff8133676f>] __blk_end_request_all+0x1f/0x30
 [<ffffffff81480a20>] virtblk_done+0x100/0x260
 [<ffffffff81412b11>] vring_interrupt+0x31/0x50
 [<ffffffff810a6394>] handle_irq_event_percpu+0x54/0x1f0
 [<ffffffff810a6578>] handle_irq_event+0x48/0x70
 [<ffffffff810a8de7>] handle_edge_irq+0x77/0x110
 [<ffffffff810044de>] handle_irq+0x1e/0x30
 [<ffffffff816f70ba>] do_IRQ+0x5a/0xe0
 [<ffffffff816ecdaf>] common_interrupt+0x6f/0x6f
 <EOI>  [<ffffffff810be18e>] ? lock_acquire+0x9e/0x100
 [<ffffffff811a76ad>] ? path_init+0x26d/0x4c0
 [<ffffffff8107ca94>] lg_local_lock+0x34/0x70
 [<ffffffff811a76ad>] ? path_init+0x26d/0x4c0
 [<ffffffff811a76ad>] path_init+0x26d/0x4c0
 [<ffffffff81370898>] ? __raw_spin_lock_init+0x38/0x70
 [<ffffffff811abbb1>] path_openat+0x81/0x660
 [<ffffffff81165031>] ? handle_mm_fault+0x2d1/0xaf0
 [<ffffffff816f0638>] ? __do_page_fault+0x148/0x570
 [<ffffffff811ac4b3>] do_filp_open+0x43/0xa0
 [<ffffffff816ec897>] ? _raw_spin_unlock+0x27/0x30
 [<ffffffff811b9876>] ? __alloc_fd+0xd6/0x130
 [<ffffffff8119a62c>] do_sys_open+0x13c/0x230
 [<ffffffff8119a742>] SyS_open+0x22/0x30
 [<ffffffff816f5192>] system_call_fastpath+0x16/0x1b
---[ end trace bd3f4c22c85fc732 ]---

The test itself concludes successfully as written, though it's worth noting
the test does not check the integrity of the data written.

The same warning can be triggered by the following simple sequence of
commands, extracted from ext4/271:

mkfs.ext4 /dev/vdc
mount -o dioread_nolock,noload /dev/vdc /vdc
touch /vdc/file
chattr +S /vdc/file
dd if=/dev/zero of=/vdc/file bs=4k count=1

(FWIW, if you omit the touch and chattr commands, the warning won't be
triggered unless a second instance of the dd command is issued.)

The warning bisects cleanly to this patch:
7b7a8665ed direct-io: Implement generic deferred AIO completions

Apparently, !io_end->handle is unexpectedly true in this test case.  A
WARN_ON for that condition was newly added in the patch.

It's not yet clear to me how serious this warning is, but the new message is
at least a minor regression.

Thanks,
Eric