I've accidentally mounted one overlayfs over another and got obvious
warning from lockdep: i_mutex lockdep classes are per-fs-type.
# mount -t overlay overlay 1 -o
upperdir=1_upper,workdir=1_work,lowerdir=1_lower
# mount -t overlay overlay 2 -o upperdir=2_upper,workdir=2_work,lowerdir=1
# ls 2
[ 305.098705] =============================================
[ 305.099011] [ INFO: possible recursive locking detected ]
[ 305.099011] 4.1.0+ #76 Not tainted
[ 305.099011] ---------------------------------------------
[ 305.099011] ls/3070 is trying to acquire lock:
[ 305.099011] (&sb->s_type->i_mutex_key#13){+.+.+.}, at:
[<ffffffff811b9ac0>] iterate_dir+0x70/0x130
[ 305.099011]
[ 305.099011] but task is already holding lock:
[ 305.099011] (&sb->s_type->i_mutex_key#13){+.+.+.}, at:
[<ffffffff811b9ac0>] iterate_dir+0x70/0x130
[ 305.099011]
[ 305.099011] other info that might help us debug this:
[ 305.099011] Possible unsafe locking scenario:
[ 305.099011]
[ 305.099011] CPU0
[ 305.099011] ----
[ 305.099011] lock(&sb->s_type->i_mutex_key#13);
[ 305.099011] lock(&sb->s_type->i_mutex_key#13);
[ 305.099011]
[ 305.099011] *** DEADLOCK ***
[ 305.099011]
[ 305.099011] May be due to missing lock nesting notation
[ 305.099011]
[ 305.099011] 1 lock held by ls/3070:
[ 305.099011] #0: (&sb->s_type->i_mutex_key#13){+.+.+.}, at:
[<ffffffff811b9ac0>] iterate_dir+0x70/0x130
[ 305.099011]
[ 305.099011] stack backtrace:
[ 305.099011] CPU: 2 PID: 3070 Comm: ls Not tainted 4.1.0+ #76
[ 305.099011] Hardware name: OpenStack Foundation OpenStack Nova, BIOS
Bochs 01/01/2011
[ 305.099011] ffff880037f40000 ffff880037fa3b48 ffffffff81991179
0000000000000007
[ 305.099011] ffffffff825ac1a0 ffff880037fa3c38 ffffffff810a92ea
0000000000000000
[ 305.099011] 0000000000000000 ffff8802110eaea0 ffff88021118c720
ffff880214576710
[ 305.099011] Call Trace:
[ 305.099011] [<ffffffff81991179>] dump_stack+0x4c/0x65
[ 305.099011] [<ffffffff810a92ea>] __lock_acquire+0x91a/0x1f60
[ 305.099011] [<ffffffff811d0a1b>] ? inode_to_bdi+0x2b/0x60
[ 305.099011] [<ffffffff810ab0d1>] lock_acquire+0xd1/0x290
[ 305.099011] [<ffffffff811b9ac0>] ? iterate_dir+0x70/0x130
[ 305.099011] [<ffffffff8199b2c8>] mutex_lock_killable_nested+0x68/0x440
[ 305.099011] [<ffffffff811b9ac0>] ? iterate_dir+0x70/0x130
[ 305.099011] [<ffffffff811b9ac0>] ? iterate_dir+0x70/0x130
[ 305.099011] [<ffffffff811b9ac0>] iterate_dir+0x70/0x130
[ 305.099011] [<ffffffff812fcf05>] ovl_dir_read_merged+0x125/0x160
[ 305.099011] [<ffffffff812fcc10>] ? ovl_cache_entry_new+0x110/0x110
[ 305.099011] [<ffffffff812fd45d>] ovl_iterate+0x14d/0x1e0
[ 305.099011] [<ffffffff811b9afc>] iterate_dir+0xac/0x130
[ 305.099011] [<ffffffff811c4d64>] ? __fget_light+0x74/0xa0
[ 305.099011] [<ffffffff811b9c87>] SyS_getdents+0x87/0x100
[ 305.099011] [<ffffffff811b9920>] ? filldir64+0x140/0x140
[ 305.099011] [<ffffffff8199e5ae>] entry_SYSCALL_64_fastpath+0x12/0x76
--
Konstantin
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