I've observed this hanging task now several times. Not sure when this started, but 2.6.32 is affected too, IIRC. I don't have a test pattern for this. Dovecot imap triggers this from time to time. I've enabled CONFIG_DETECT_HUNG_TASK now and got this two tasks which hang:
INFO: task imap:2958 blocked for more than 120 seconds.
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
imap D 0000000000000000 0 2958 2653 0x00000000
ffff88008caf5a28 0000000000000046 0000000000000000 ffffffff810544cf
ffff88008caf5998 0000000000000001 ffff88008caf5fd8 ffff88008caf9530
000000000000de78 00000000001d2700 00000000001d2700 ffff88008caf9530
Call Trace:
[<ffffffff810544cf>] ? trace_hardirqs_off+0xd/0xf
[<ffffffff8105613b>] ? trace_hardirqs_on_caller+0x10c/0x130
[<ffffffff81071262>] ? sync_page+0x0/0x48
[<ffffffff8138b46a>] io_schedule+0x38/0x4d
[<ffffffff810712a6>] sync_page+0x44/0x48
[<ffffffff8138b99a>] __wait_on_bit_lock+0x41/0x8a
[<ffffffff8107124e>] __lock_page+0x61/0x68
[<ffffffff810475a3>] ? wake_bit_function+0x0/0x2e
[<ffffffff810718c6>] filemap_fault+0xea/0x345
[<ffffffff81084022>] __do_fault+0x50/0x3d3
[<ffffffff81086077>] handle_mm_fault+0x32f/0x65d
[<ffffffff8102039d>] ? do_page_fault+0xf4/0x26f
[<ffffffff811cacb4>] ? __down_read_trylock+0x46/0x4e
[<ffffffff8104b0f3>] ? down_read_trylock+0x3f/0x49
[<ffffffff8102039d>] ? do_page_fault+0xf4/0x26f
[<ffffffff81020500>] do_page_fault+0x257/0x26f
[<ffffffff8138e2ff>] page_fault+0x1f/0x30
[<ffffffff81083a13>] ? might_fault+0x57/0xa7
[<ffffffff811746e2>] ? btrfs_copy_from_user+0x4f/0x113
[<ffffffff81174771>] ? btrfs_copy_from_user+0xde/0x113
[<ffffffff81174cd0>] btrfs_file_write+0x439/0x6fe
[<ffffffff810a5c34>] vfs_write+0xad/0x14e
[<ffffffff8105613b>] ? trace_hardirqs_on_caller+0x10c/0x130
[<ffffffff810a5d2a>] sys_pwrite64+0x55/0x74
[<ffffffff81001eeb>] system_call_fastpath+0x16/0x1b
2 locks held by imap/2958:
#0: (&sb->s_type->i_mutex_key#4){+.+.+.}, at: [<ffffffff81174a00>]
btrfs_file_write+0x169/0x6fe
#1: (&mm->mmap_sem){++++++}, at: [<ffffffff8102039d>]
do_page_fault+0xf4/0x26f
INFO: task flush-btrfs-2:2783 blocked for more than 120 seconds.
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
flush-btrfs-2 D 0000000000000000 0 2783 2 0x00000000
ffff88010cdcf9d0 0000000000000046 0000000000000000 ffffffff810544cf
ffff88010cdcf940 0000000000000000 ffff88010cdcffd8 ffff88010cd18290
000000000000de78 00000000001d2700 00000000001d2700 ffff88010cd18290
Call Trace:
[<ffffffff810544cf>] ? trace_hardirqs_off+0xd/0xf
[<ffffffff8105613b>] ? trace_hardirqs_on_caller+0x10c/0x130
[<ffffffff81071262>] ? sync_page+0x0/0x48
[<ffffffff8138b46a>] io_schedule+0x38/0x4d
[<ffffffff810712a6>] sync_page+0x44/0x48
[<ffffffff8138b99a>] __wait_on_bit_lock+0x41/0x8a
[<ffffffff81070ce1>] ? find_get_pages_tag+0x0/0x130
[<ffffffff8107124e>] __lock_page+0x61/0x68
[<ffffffff810475a3>] ? wake_bit_function+0x0/0x2e
[<ffffffff811871f6>] T.858+0xf1/0x2cd
[<ffffffff8104c4f9>] ? sched_clock_cpu+0xc6/0xd4
[<ffffffff8104c3cd>] ? sched_clock_local+0x1c/0x82
[<ffffffff8104c4f9>] ? sched_clock_cpu+0xc6/0xd4
[<ffffffff810544cf>] ? trace_hardirqs_off+0xd/0xf
[<ffffffff811874d3>] extent_writepages+0x3f/0x54
[<ffffffff8116dbd5>] ? btrfs_get_extent+0x0/0x7ee
[<ffffffff8116da7e>] btrfs_writepages+0x22/0x24
[<ffffffff81077d12>] do_writepages+0x1f/0x28
[<ffffffff810c0233>] writeback_single_inode+0xf1/0x2f0
[<ffffffff810c0f57>] writeback_inodes_wb+0x3a9/0x4b2
[<ffffffff810c118b>] wb_writeback+0x12b/0x1af
[<ffffffff810c1436>] wb_do_writeback+0x17f/0x195
[<ffffffff810c1342>] ? wb_do_writeback+0x8b/0x195
[<ffffffff810c1477>] bdi_writeback_task+0x2b/0x84
[<ffffffff81082a45>] ? bdi_start_fn+0x0/0xcf
[<ffffffff81082ab6>] bdi_start_fn+0x71/0xcf
[<ffffffff81082a45>] ? bdi_start_fn+0x0/0xcf
[<ffffffff81047197>] kthread+0x7a/0x82
[<ffffffff81002c94>] kernel_thread_helper+0x4/0x10
[<ffffffff8138e0fc>] ? restore_args+0x0/0x30
[<ffffffff8104711d>] ? kthread+0x0/0x82
[<ffffffff81002c90>] ? kernel_thread_helper+0x0/0x10
1 lock held by flush-btrfs-2/2783:
#0: (&type->s_umount_key#20){++++..}, at: [<ffffffff810c0e82>]
writeback_inodes_wb+0x2d4/0x4b2
regards,
Johannes
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