I can reliably get btrfs to panic by running my fs_mark code on a newly 
created file system with lots of threads on an 8-way box. If this is too 
aggressive, let me know ;-)

Here is a summary of the panic:

EXT3-fs: recovery complete.
EXT3-fs: mounted filesystem with ordered data mode.
device fsid 814e6131acbfcbec-7a2a40df880929bb devid 1 transid 495 /dev/sdb1
BUG: soft lockup - CPU#1 stuck for 61s! [fs_mark:4572]
CPU 1:
Modules linked in: btrfs libcrc32c ipt_MASQUERADE iptable_nat nf_nat 
bridge bnep rfcomm l2cap bluetooth ib_iser rdma_cm ib_cm iw_cm ib_sa 
ib_mad ib_core ib_addr iscsi_tcp libiscsi scsi_transport_iscsi fuse 
sunrpc ipt_REJECT nf_conntrack_ipv4 iptable_filter ip_tables ip6t_REJECT 
xt_tcpudp nf_conntrack_ipv6 xt_state nf_conntrack ip6table_filter 
ip6_tables x_tables ipv6 dm_mirror dm_multipath dm_mod kvm_intel kvm 
snd_hda_intel snd_seq_dummy snd_seq_oss snd_seq_midi_event snd_seq 
snd_seq_device snd_pcm_oss iTCO_wdt snd_mixer_oss iTCO_vendor_support 
nvidia(P) pata_acpi button ata_piix snd_pcm ppdev firewire_ohci i2c_i801 
ata_generic firewire_core pcspkr dcdbas snd_timer sr_mod cdrom 
snd_page_alloc snd_hwdep snd tg3 serio_raw i2c_core shpchp crc_itu_t sg 
parport_pc soundcore parport floppy ahci libata sd_mod scsi_mod ext3 jbd 
mbcache uhci_hcd ohci_hcd ehci_hcd [last unloaded: freq_table]
Pid: 4572, comm: fs_mark Tainted: P         2.6.25.3-18.fc9.x86_64 #1
RIP: 0010:[<ffffffff81132929>]  [<ffffffff81132929>] 
__write_lock_failed+0x9/0x20
RSP: 0018:ffff81000c529e60  EFLAGS: 00000206
RAX: ffff810015c0e000 RBX: ffff81000c529e68 RCX: 0000000000000016
RDX: ffff81003d019e00 RSI: 0000000000000001 RDI: ffff8100100e24f0
RBP: ffff8100189bef00 R08: 0000000000000000 R09: 0000000000000016
R10: 0000000012750e57 R11: 0000000000000246 R12: 0000000000000202
R13: ffff81000c529de8 R14: 00000000000011dc R15: ffff81000c529f58
FS:  000000000159b850(0063) GS:ffff81003f802680(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
CR2: 00007f55301940a8 CR3: 000000000c513000 CR4: 00000000000026e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400

Call Trace:
 [<ffffffff81292408>] ? _write_lock+0x12/0x14
 [<ffffffff88c27ff2>] ? :btrfs:btrfs_del_ordered_inode+0xc0/0x13f
 [<ffffffff88c1e67f>] ? :btrfs:btrfs_release_file+0x9/0xd
 [<ffffffff810a5eff>] ? __fput+0xca/0x189
 [<ffffffff810a5fd2>] ? fput+0x14/0x16
 [<ffffffff810a32c8>] ? filp_close+0x66/0x71
 [<ffffffff810a336c>] ? sys_close+0x99/0xd2
 [<ffffffff8100c052>] ? tracesys+0xd5/0xda


ric