Thanks Mikulas, Good analysis. Looking at this now. On Thu, Oct 20, 2011 at 02:59:12PM -0400, Mikulas Patocka wrote: > Hi Joe > > Zdenek recently got a lockdep warning in dm-thin. See this: > http://pastebin.test.redhat.com/65346 > > I looked at the warning and realized that it is not a false alarm, but a > real bug. > > Lockdep warns about dependency &md->io_lock -> &md->map_lock, but io_lock > is a semaphore and map_lock is a spinlock, so such lock ordering should be > always allowed (you can't take them in reverse order, because you can't > take a semaphore while holding a spinlock). > > Looking deeper in the warning, it complains that there is an exeisting > dependency chain &md->map_lock --> &pmd->root_lock --> &md->io_lock > > Here, map_lock is a spinlock and root_lock is a semaphore, this order of > taking locks is obviously wrong. > > Cutting relevant part of the backtrace, we see: > 41. [ 3836.656384] -> #1 (&pmd->root_lock){+++++.}: > 42. [ 3836.656384] [<ffffffff81090740>] lock_acquire+0x90/0x1f0 > 43. [ 3836.656384] [<ffffffff814ac1ce>] down_read+0x3e/0x86 > 44. [ 3836.656384] [<ffffffffa02aaafe>] > dm_pool_get_data_dev_size+0x2e/0x60 [dm_thin_pool] > 45. [ 3836.656384] [<ffffffffa02a615b>] > thin_iterate_devices+0x3b/0x80 [dm_thin_pool] > 46. [ 3836.656384] [<ffffffffa006b2bf>] > dm_table_supports_flush+0x5f/0x90 [dm_mod] > 47. [ 3836.656384] [<ffffffffa006d2ec>] > dm_table_set_restrictions+0x9c/0x180 [dm_mod] > 48. [ 3836.656384] [<ffffffffa006a88a>] dm_swap_table+0x1da/0x2e0 > [dm_mod] > 49. [ 3836.656384] [<ffffffffa00701db>] dev_suspend+0x11b/0x260 > [dm_mod] > 50. [ 3836.656384] [<ffffffffa007089a>] ctl_ioctl+0x15a/0x2c0 > [dm_mod] > > dm_swap_table calls __bind (which is not shown because it is inlined), > __bind takes a spinlock &md->map_lock and calls dm_table_set_restrictions, > from there, the code goes to thin_iterate_devices and it tries to take a > semaphore &pmd->root_lock. > > So there is a clear bug, dm-thin is taking a semaphore while spinlock > &md->map_lock is held. > > Mikulas -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
