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    Subjectlockdep warning when nesting dm devices
    FromPeter Osterlund <>
    Date15 Jul 2006 13:17:29 +0200
    If I run this as root:
    
    # dd if=/dev/zero of=tmpfile bs=1M count=10
    # /sbin/losetup -f ./tmpfile
    # echo "0 10000 linear /dev/loop0 0" | /sbin/dmsetup create test
    # echo "0 10000 linear /dev/mapper/test 0" | /sbin/dmsetup create test2
    I get the following warning from the lockdep validator.
    
    Btw, is there a limit on how many dm devices can be chained? I guess
    there will be a kernel stack overflow if you try to chain together too
    many devices.
    
    =============================================
    [ INFO: possible recursive locking detected ]
    ---------------------------------------------
    vol_id/2468 is trying to acquire lock:
     (&md->io_lock){----}, at: [<e08475b6>] dm_request+0x23/0x158 [dm_mod]
    but task is already holding lock:
     (&md->io_lock){----}, at: [<e08475b6>] dm_request+0x23/0x158 [dm_mod]
    other info that might help us debug this:
    1 lock held by vol_id/2468:
     #0:  (&md->io_lock){----}, at: [<e08475b6>] dm_request+0x23/0x158 [dm_mod]
    stack backtrace:
     [<c01041bc>] show_trace_log_lvl+0x133/0x14d
     [<c01048e4>] show_trace+0x1b/0x1d
     [<c01049b2>] dump_stack+0x26/0x28
     [<c0134b92>] __lock_acquire+0x8aa/0xd29
     [<c013535e>] lock_acquire+0x68/0x83
     [<c01314ff>] down_read+0x50/0x60
     [<e08475b6>] dm_request+0x23/0x158 [dm_mod]
     [<c01e02e8>] generic_make_request+0x94/0x322
     [<e084649a>] __map_bio+0x75/0x130 [dm_mod]
     [<e08472e0>] __split_bio+0x382/0x3c1 [dm_mod]
     [<e0847670>] dm_request+0xdd/0x158 [dm_mod]
     [<c01e02e8>] generic_make_request+0x94/0x322
     [<c01e2a3b>] submit_bio+0x6f/0x112
     [<c01647a1>] submit_bh+0xce/0x11f
     [<c0167902>] block_read_full_page+0x26c/0x2e2
     [<c016a74a>] blkdev_readpage+0x19/0x1b
     [<c014c5ba>] __do_page_cache_readahead+0x1bf/0x28a
     [<c014c6e2>] blockable_page_cache_readahead+0x5d/0xc4
     [<c014c961>] page_cache_readahead+0x176/0x1bc
     [<c01466c7>] do_generic_mapping_read+0x49d/0x50d
     [<c0147084>] __generic_file_aio_read+0xfd/0x22c
     [<c0148491>] generic_file_read+0x9e/0xbe
     [<c0163460>] vfs_read+0x98/0x115
     [<c0163869>] sys_read+0x47/0x6e
     [<c0103069>] sysenter_past_esp+0x56/0x8d
     [<b7f79410>] 0xb7f79410
    
    -- 
    Peter Osterlund - petero2@telia.com
    http://web.telia.com/~u89404340
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