INFO: task hung in lookup_slow

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit: 99055915 ANDROID: sdcardfs: Check stacked filesystem depth
git tree: android-4.9
console output: https://syzkaller.appspot.com/x/log.txt?x=1557c252400000
kernel config: https://syzkaller.appspot.com/x/.config?x=f2dbd0c9dd968786
dashboard link: https://syzkaller.appspot.com/bug?extid=74fcd191c4cd336f12e4
compiler: gcc (GCC) 8.0.1 20180413 (experimental)
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=14bb408c400000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+74fcd1...@syzkaller.appspotmail.com

IPv6: ADDRCONF(NETDEV_CHANGE): veth0: link becomes ready
INFO: task syz-executor0:4386 blocked for more than 140 seconds.
Not tainted 4.9.113-g9905591 #14
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
syz-executor0 D29144 4386 3910 0x00000004
ffff8801cf481800 0000000000000000 ffff8801ce2e8000 ffffffff84629800
ffff8801db221c18 ffff8801ba13f680 ffffffff839e929d ffffffff81235647
0000000000000000 ffff8801cf4820c0 0000000600000007 ffff8801db2224e8
Call Trace:
[<ffffffff839ea89f>] schedule+0x7f/0x1b0 kernel/sched/core.c:3553
[<ffffffff839f4cc4>] rwsem_down_read_failed+0x1e4/0x320
kernel/locking/rwsem-xadd.c:260
[<ffffffff81ee7558>] call_rwsem_down_read_failed+0x18/0x30
arch/x86/lib/rwsem.S:94
[<ffffffff839f3bf2>] __down_read arch/x86/include/asm/rwsem.h:65 [inline]
[<ffffffff839f3bf2>] down_read+0x52/0xb0 kernel/locking/rwsem.c:24
[<ffffffff81594cb1>] inode_lock_shared include/linux/fs.h:776 [inline]
[<ffffffff81594cb1>] lookup_slow+0x141/0x470 fs/namei.c:1645
[<ffffffff815a1336>] walk_component+0x716/0x1bb0 fs/namei.c:1784
[<ffffffff815a3c96>] lookup_last fs/namei.c:2266 [inline]
[<ffffffff815a3c96>] path_lookupat.isra.41+0x186/0x410 fs/namei.c:2283
[<ffffffff815a9747>] filename_lookup.part.55+0x177/0x370 fs/namei.c:2317
[<ffffffff815a9b03>] filename_lookup fs/namei.c:2310 [inline]
[<ffffffff815a9b03>] user_path_at_empty+0x53/0x70 fs/namei.c:2578
[<ffffffff81580ae6>] user_path_at include/linux/namei.h:55 [inline]
[<ffffffff81580ae6>] vfs_fstatat+0xc6/0x170 fs/stat.c:106
[<ffffffff81581d89>] vfs_lstat fs/stat.c:129 [inline]
[<ffffffff81581d89>] SYSC_newlstat+0x89/0x100 fs/stat.c:283
[<ffffffff815820cd>] SyS_newlstat+0x1d/0x30 fs/stat.c:277
[<ffffffff81006316>] do_syscall_64+0x1a6/0x490 arch/x86/entry/common.c:282
[<ffffffff839fa193>] entry_SYSCALL_64_after_swapgs+0x5d/0xdb

Showing all locks held in the system:
2 locks held by khungtaskd/519:
#0: (rcu_read_lock){......}, at: [<ffffffff8136675c>]
check_hung_uninterruptible_tasks kernel/hung_task.c:168 [inline]
#0: (rcu_read_lock){......}, at: [<ffffffff8136675c>]
watchdog+0x11c/0xa20 kernel/hung_task.c:239
#1: (tasklist_lock){.+.+..}, at: [<ffffffff81425de7>]
debug_show_all_locks+0x79/0x218 kernel/locking/lockdep.c:4336
2 locks held by getty/3761:
#0: (&tty->ldisc_sem){++++++}, at: [<ffffffff839f8342>]
ldsem_down_read+0x32/0x40 drivers/tty/tty_ldsem.c:367
#1: (&ldata->atomic_read_lock){+.+...}, at: [<ffffffff8211ee22>]
n_tty_read+0x202/0x16e0 drivers/tty/n_tty.c:2142
1 lock held by syz-executor0/4386:
#0: (&type->i_mutex_dir_key#4){++++++}, at: [<ffffffff81594cb1>]
inode_lock_shared include/linux/fs.h:776 [inline]
#0: (&type->i_mutex_dir_key#4){++++++}, at: [<ffffffff81594cb1>]
lookup_slow+0x141/0x470 fs/namei.c:1645
1 lock held by syz-executor0/4388:
#0: (&type->i_mutex_dir_key#4){++++++}, at: [<ffffffff81594cb1>]
inode_lock_shared include/linux/fs.h:776 [inline]
#0: (&type->i_mutex_dir_key#4){++++++}, at: [<ffffffff81594cb1>]
lookup_slow+0x141/0x470 fs/namei.c:1645
2 locks held by syz-executor0/4389:
#0: (sb_writers#10){.+.+.+}, at: [<ffffffff815e0a8f>] sb_start_write
include/linux/fs.h:1573 [inline]
#0: (sb_writers#10){.+.+.+}, at: [<ffffffff815e0a8f>]
mnt_want_write+0x3f/0xb0 fs/namespace.c:391
#1: (&type->i_mutex_dir_key#4){++++++}, at: [<ffffffff81568e21>]
inode_lock include/linux/fs.h:766 [inline]
#1: (&type->i_mutex_dir_key#4){++++++}, at: [<ffffffff81568e21>]
chmod_common+0x171/0x450 fs/open.c:543

=============================================

NMI backtrace for cpu 0
CPU: 0 PID: 519 Comm: khungtaskd Not tainted 4.9.113-g9905591 #14
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
Google 01/01/2011
ffff8801d8587d08 ffffffff81eb32a9 0000000000000000 0000000000000000
0000000000000000 0000000000000001 ffffffff810b9bd0 ffff8801d8587d40
ffffffff81ebe5a7 0000000000000000 0000000000000000 0000000000000003
Call Trace:
[<ffffffff81eb32a9>] __dump_stack lib/dump_stack.c:15 [inline]
[<ffffffff81eb32a9>] dump_stack+0xc1/0x128 lib/dump_stack.c:51
[<ffffffff81ebe5a7>] nmi_cpu_backtrace.cold.2+0x48/0x87
lib/nmi_backtrace.c:99
[<ffffffff81ebe53a>] nmi_trigger_cpumask_backtrace+0x12a/0x14f
lib/nmi_backtrace.c:60
[<ffffffff810b9cd4>] arch_trigger_cpumask_backtrace+0x14/0x20
arch/x86/kernel/apic/hw_nmi.c:37
[<ffffffff81366cf4>] trigger_all_cpu_backtrace include/linux/nmi.h:58
[inline]
[<ffffffff81366cf4>] check_hung_task kernel/hung_task.c:125 [inline]
[<ffffffff81366cf4>] check_hung_uninterruptible_tasks
kernel/hung_task.c:182 [inline]
[<ffffffff81366cf4>] watchdog+0x6b4/0xa20 kernel/hung_task.c:239
[<ffffffff8119d0ad>] kthread+0x26d/0x300 kernel/kthread.c:211
[<ffffffff839fa35c>] ret_from_fork+0x5c/0x70 arch/x86/entry/entry_64.S:373
Sending NMI from CPU 0 to CPUs 1:
NMI backtrace for cpu 1
CPU: 1 PID: 4385 Comm: syz-executor0 Not tainted 4.9.113-g9905591 #14
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
Google 01/01/2011
task: ffff8801cf97b000 task.stack: ffff8801ba130000
RIP: 0010:[<ffffffff812215e9>] c [<ffffffff812215e9>] constant_test_bit
arch/x86/include/asm/bitops.h:311 [inline]
RIP: 0010:[<ffffffff812215e9>] c [<ffffffff812215e9>] test_ti_thread_flag
include/linux/thread_info.h:63 [inline]
RIP: 0010:[<ffffffff812215e9>] c [<ffffffff812215e9>] test_tsk_thread_flag
include/linux/sched.h:3439 [inline]
RIP: 0010:[<ffffffff812215e9>] c [<ffffffff812215e9>] signal_pending
include/linux/sched.h:3465 [inline]
RIP: 0010:[<ffffffff812215e9>] c [<ffffffff812215e9>] signal_pending_state
include/linux/sched.h:3482 [inline]
RIP: 0010:[<ffffffff812215e9>] c [<ffffffff812215e9>]
prepare_to_wait_event+0x99/0x450 kernel/sched/wait.c:214
RSP: 0018:ffff8801ba1377b8 EFLAGS: 00000046
RAX: 0000000000000001 RBX: 1ffff10037426efa RCX: dffffc0000000000
RDX: ffff8801cf97b000 RSI: 1ffff10039f2f600 RDI: ffff8801cec6a564
RBP: ffff8801ba137858 R08: 0000000000000286 R09: 0000000000000001
R10: ffff8801cf97b000 R11: 1ffff10039f2f722 R12: ffff8801ba137830
R13: ffff8801ba1378b0 R14: ffff8801cec6a560 R15: 0000000000000001
FS: 00007f7956f7e700(0000) GS:ffff8801db300000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007fc5e01d9280 CR3: 00000001ceee2000 CR4: 00000000001606f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Stack:
ffff8801ba1377e0 c 0000000000000246 c ffff8801cf97b000 c 0000000041b58ab3 c
ffffffff843bbae3 c ffffffff81221550 c 0000000000000246 c ffff8801cf97b000 c
ffffffff83db4480 c 00000000000001cc c 0000000000000000 c ffff8801cec6a4c0 c
Call Trace:
[<ffffffff81907bbe>] request_wait_answer+0x22e/0x6e0 fs/fuse/dev.c:459
[<ffffffff819086c9>] __fuse_request_send+0x109/0x1b0 fs/fuse/dev.c:480
[<ffffffff819087cd>] fuse_request_send+0x5d/0x70 fs/fuse/dev.c:493
[<ffffffff81912a24>] fuse_simple_request+0x2f4/0x660 fs/fuse/dev.c:551
[<ffffffff8191cffb>] fuse_do_setattr+0x68b/0x11e0 fs/fuse/dir.c:1722
[<ffffffff8191dd42>] fuse_setattr+0x1f2/0x510 fs/fuse/dir.c:1822
[<ffffffff815d3f90>] notify_change2+0xac0/0xdf0 fs/attr.c:313
[<ffffffff81568ec7>] chmod_common+0x217/0x450 fs/open.c:549
[<ffffffff8156ccfd>] SYSC_fchmodat fs/open.c:582 [inline]
[<ffffffff8156ccfd>] SyS_fchmodat fs/open.c:574 [inline]
[<ffffffff8156ccfd>] SYSC_chmod fs/open.c:594 [inline]
[<ffffffff8156ccfd>] SyS_chmod+0xad/0x130 fs/open.c:592
[<ffffffff81006316>] do_syscall_64+0x1a6/0x490 arch/x86/entry/common.c:282
[<ffffffff839fa193>] entry_SYSCALL_64_after_swapgs+0x5d/0xdb
Code: c14 c25 cc0 c7d c01 c00 c41 cf6 cc7 c81 c49 c89 cc0 c49
c63 cc7 c0f c84 cc4 c00 c00 c00 c48 cb9 c00 c00 c00 c00 c00
cfc cff cdf c48 c89 cd6 c48 cc1 cee c03 c80 c3c c0e c00
c<0f> c85 cba c02 c00 c00 c48 c8b c0a c83 ce1 c04 c0f c84
c9d c00 c00 c00 c41 c83 ce7 c


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