Hi,
we encountered a deadlock inside the kernel part of the device-mapper
code. it was found in a CentOS 5.3 system's kernel - but from looking at
the code of kernel 2.6.31 - the same bug is still in there.
below is the stack trace of the self-deadlocking code. this is one of
the threads of multipathd, that attempts to remove a dm device using a
ioctl to the dm driver:
crash> bt 22619
PID: 22619 TASK: ffff8106521247e0 CPU: 3 COMMAND: "multipathd"
#0 [ffff8106298dfb48] schedule at ffffffff80063035
#1 [ffff8106298dfc20] __down_read at ffffffff8006475d
#2 [ffff8106298dfc60] dm_copy_name_and_uuid at ffffffff8824f740
#3 [ffff8106298dfc90] dm_send_uevents at ffffffff88252685
#4 [ffff8106298dfcd0] event_callback at ffffffff8824c678
#5 [ffff8106298dfd00] dm_table_event at ffffffff8824dd01
#6 [ffff8106298dfd10] __hash_remove at ffffffff882507ad
#7 [ffff8106298dfd30] dev_remove at ffffffff88250865
#8 [ffff8106298dfd60] ctl_ioctl at ffffffff88250d80
#9 [ffff8106298dfee0] do_ioctl at ffffffff800418c4
#10 [ffff8106298dff00] vfs_ioctl at ffffffff8002fab9
#11 [ffff8106298dff40] sys_ioctl at ffffffff8004bdaf
#12 [ffff8106298dff80] tracesys at ffffffff8005d28d (via system_call)
RIP: 00000039deecbb47 RSP: 0000000041e35bb8 RFLAGS: 00000246
RAX: ffffffffffffffda RBX: ffffffff8005d28d RCX: ffffffffffffffff
RDX: 000000001b9a7ac0 RSI: 00000000c138fd04 RDI: 0000000000000007
RBP: 0000000000000000 R8: 00000039df211e45 R9: 000000001b9a7af0
R10: 00000039df211d59 R11: 0000000000000246 R12: 00000039df211e23
R13: 0000000000000000 R14: 00000039df211d59 R15: 0000000000000000
ORIG_RAX: 0000000000000010 CS: 0033 SS: 002b
(note: the crash was taken using kdump).
the problem appears to be that the function dm_remove in file
drivers/md/dm-ioctl.c is locking the _hash_lock rw semaphore for write
(down_write(&_hash_lock);), and then later in the call chain, the
function dm_copy_name_and_uuid (in the same source file) attempts to
lock the same semaphore for read. since the semaphore is not recursive -
there is a deadlock. naturally, when this happens, any command trying to
access those data structures (dmsetup, multipath, etc) block as well.
if my analysis is correct - is there any idea on how to go about fixing
this? i can see several diffeernt paths - one is to store the data
required by dm_copy_name_and_uuid in a location that won't require
locking - or altenatively, have a dual version of the relevant functions
- one to be invoked when there lock is not held, and one to be invoked
when the lock is held.
note: we've encountered this deadlock twice in the past week - no idea
if we saw it in the past or not.
thanks,
--guy
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