zpl_writepage() deadlock

[behlendorf]

"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
bdi-default   D 0000000000000001     0    49      2 0x00000000
 ffff88007cecad90 0000000000000046 0000000000000000 ffffffff8100ae96
 ffffffff81a5fa40 0000000000001308 ffff880005514800 00000001006232d5
 ffff88007cec70b8 ffff88007cecbfd8 000000000000f598 ffff88007cec70b8
Call Trace:
 [] ? ftrace_call+0x5/0x2b
 [] rwsem_down_failed_common+0x95/0x1d0
 [] rwsem_down_write_failed+0x23/0x30
 [] call_rwsem_down_write_failed+0x13/0x20
 [] ? down_write+0x32/0x40
 [] dnode_new_blkid+0x94/0x2d0 [zfs]
 [] dbuf_dirty+0x556/0x750 [zfs]
 [] dmu_buf_will_dirty+0x81/0xd0 [zfs]
 [] dmu_write+0x90/0x170 [zfs]
 [] zfs_putpage+0x2ce/0x360 [zfs]
 [] zpl_putpage+0x1e/0x60 [zfs]
 [] zpl_writepage+0x12/0x20 [zfs]
 [] writeout+0xa7/0xd0
 [] move_to_new_page+0x13b/0x170
 [] migrate_pages+0x434/0x4c0
 [] ? compaction_alloc+0x0/0x3e0
 [] compact_zone+0x4fb/0x780
 [] ? ftrace_call+0x5/0x2b
 [] compact_zone_order+0xa1/0xe0
 [] try_to_compact_pages+0x11c/0x190
 [] __alloc_pages_nodemask+0x5eb/0x8b0
 [] alloc_pages_current+0xaa/0x110
 [] __get_free_pages+0xe/0x50
 [] kv_alloc+0x3f/0xb0 [spl]
 [] spl_kmem_cache_alloc+0x339/0x660 [spl]
 [] dbuf_create+0x43/0x370 [zfs]
 [] ? mutex_unlock+0x9/0x20
 [] ? dbuf_find+0xe0/0x100 [zfs]
 [] __dbuf_hold_impl+0x241/0x480 [zfs]
 [] dbuf_hold_impl+0x86/0xc0 [zfs]
 [] dbuf_hold_level+0x1f/0x30 [zfs]
 [] dmu_tx_check_ioerr+0x4e/0x110 [zfs]
 [] dmu_tx_count_write+0x359/0x6f0 [zfs]
 [] ? ftrace_call+0x5/0x2b
 [] ? _cond_resched+0x9/0x40
 [] ? __kmalloc+0x20c/0x220
 [] ? kmem_alloc_debug+0xb3/0x130 [spl]
 [] dmu_tx_hold_write+0x4f/0x70 [zfs]
 [] zfs_putpage+0x23d/0x360 [zfs]
 [] ? __dec_zone_state+0x9/0x80
 [] zpl_putpage+0x1e/0x60 [zfs]
 [] write_cache_pages+0x1c9/0x4a0
 [] ? zpl_putpage+0x0/0x60 [zfs]
 [] zpl_writepages+0x18/0x20 [zfs]
 [] do_writepages+0x21/0x40
 [] writeback_single_inode+0xdd/0x2c0
 [] writeback_sb_inodes+0xce/0x180
 [] writeback_inodes_wb+0xab/0x1b0
 [] wb_writeback+0x29b/0x3f0
 [] ? ftrace_call+0x5/0x2b
 [] wb_do_writeback+0xbb/0x240
 [] ? process_timeout+0x0/0x10
 [] bdi_forker_task+0x6a/0x310
 [] ? bdi_forker_task+0x0/0x310
 [] kthread+0x96/0xa0