From: syzbot <syzbot+52aa59fd5e3cd8a19dd7@syzkaller.appspotmail.com>
To: amir73il@gmail.com, linux-kernel@vger.kernel.org,
linux-unionfs@vger.kernel.org, miklos@szeredi.hu,
syzkaller-bugs@googlegroups.com
Subject: Re: [syzbot] [overlayfs?] possible deadlock in __d_alloc_parallel
Date: Thu, 15 Oct 2026 04:16:33 -0700 [thread overview]
Message-ID: <6ad0b611.5f317f00.247b6.0067.GAE@google.com> (raw)
In-Reply-To: <6abfe422.7503fbd6.2d3f6.0023.GAE@google.com>
syzbot has found a reproducer for the following issue on:
HEAD commit: d419d682257d Add linux-next specific files for 20261014
git tree: git://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git
console output: https://syzkaller.appspot.com/x/log.txt?x=1157ce8e580000
kernel config: https://syzkaller.appspot.com/x/.config?x=2935483cab352f25
dashboard link: https://syzkaller.appspot.com/bug?extid=52aa59fd5e3cd8a19dd7
compiler: Debian clang version 22.1.8 (1~deb13u4), Debian LLD 22.1.8
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=12f9d3d1580000
IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+52aa59fd5e3cd8a19dd7@syzkaller.appspotmail.com
======================================================
WARNING: possible circular locking dependency detected
syzkaller #0 Not tainted
------------------------------------------------------
syz-executor151/5964 is trying to acquire lock:
ffff88806b10d620 (DCACHE_PAR_LOOKUP){+.+.}-{0:0}, at: rcu_lock_acquire include/linux/rcupdate.h:309 [inline]
ffff88806b10d620 (DCACHE_PAR_LOOKUP){+.+.}-{0:0}, at: rcu_read_lock include/linux/rcupdate.h:849 [inline]
ffff88806b10d620 (DCACHE_PAR_LOOKUP){+.+.}-{0:0}, at: __d_alloc_parallel+0x39e/0x1700 fs/dcache.c:2883
but task is already holding lock:
ffff88805c8d39e0 (&type->i_mutex_dir_key#3){++++}-{4:4}, at: inode_lock_shared include/linux/fs.h:1039 [inline]
ffff88805c8d39e0 (&type->i_mutex_dir_key#3){++++}-{4:4}, at: lookup_slow+0x46/0x80 fs/namei.c:1936
which lock already depends on the new lock.
the existing dependency chain (in reverse order) is:
-> #1 (&type->i_mutex_dir_key#3){++++}-{4:4}:
down_read+0x45/0x320 kernel/locking/rwsem.c:1574
inode_lock_shared include/linux/fs.h:1039 [inline]
lookup_slow+0x46/0x80 fs/namei.c:1936
ovl_lookup_positive_unlocked fs/overlayfs/namei.c:216 [inline]
ovl_lookup_single+0x323/0xe90 fs/overlayfs/namei.c:260
ovl_lookup_layer+0x377/0x440 fs/overlayfs/namei.c:365
ovl_lookup_layers fs/overlayfs/namei.c:1109 [inline]
ovl_lookup+0x5e5/0x1c80 fs/overlayfs/namei.c:1403
lookup_open+0x94b/0x1970 fs/namei.c:4578
open_last_lookups fs/namei.c:4816 [inline]
path_openat+0x70c/0x20f0 fs/namei.c:5055
do_file_open+0x258/0x570 fs/namei.c:5087
do_sys_openat2+0x115/0x200 fs/open.c:1442
do_sys_open fs/open.c:1448 [inline]
__do_sys_openat fs/open.c:1788 [inline]
__se_sys_openat fs/open.c:1783 [inline]
__x64_sys_openat+0x138/0x170 fs/open.c:1783
do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
do_syscall_64+0x166/0x520 arch/x86/entry/syscall_64.c:84
entry_SYSCALL_64_after_hwframe+0x77/0x7f
-> #0 (DCACHE_PAR_LOOKUP){+.+.}-{0:0}:
check_prev_add kernel/locking/lockdep.c:3209 [inline]
check_prevs_add kernel/locking/lockdep.c:3328 [inline]
validate_chain kernel/locking/lockdep.c:3952 [inline]
__lock_acquire+0x164c/0x2de0 kernel/locking/lockdep.c:5288
lock_acquire+0x115/0x350 kernel/locking/lockdep.c:5942
d_wait_lookup fs/dcache.c:2840 [inline]
__d_alloc_parallel+0xc32/0x1700 fs/dcache.c:2944
__lookup_slow+0x82/0x2f0 fs/namei.c:1905
lookup_slow+0x59/0x80 fs/namei.c:1937
walk_component fs/namei.c:2316 [inline]
lookup_last fs/namei.c:2824 [inline]
path_lookupat+0x3f5/0x8c0 fs/namei.c:2857
filename_lookup+0x258/0x5f0 fs/namei.c:2886
do_readlinkat+0xe3/0x510 fs/stat.c:575
__do_sys_readlink fs/stat.c:609 [inline]
__se_sys_readlink fs/stat.c:606 [inline]
__x64_sys_readlink+0x7f/0x90 fs/stat.c:606
do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
do_syscall_64+0x166/0x520 arch/x86/entry/syscall_64.c:84
entry_SYSCALL_64_after_hwframe+0x77/0x7f
other info that might help us debug this:
Possible unsafe locking scenario:
CPU0 CPU1
---- ----
rlock(&type->i_mutex_dir_key#3);
lock(DCACHE_PAR_LOOKUP);
lock(&type->i_mutex_dir_key#3);
lock(DCACHE_PAR_LOOKUP);
*** DEADLOCK ***
locks held by syz-executor151/5964: 1, last CPU#1:
#0: ffff88805c8d39e0 (&type->i_mutex_dir_key#3){++++}-{4:4}, at: inode_lock_shared include/linux/fs.h:1039 [inline]
#0: ffff88805c8d39e0 (&type->i_mutex_dir_key#3){++++}-{4:4}, at: lookup_slow+0x46/0x80 fs/namei.c:1936
stack backtrace:
CPU: 1 UID: 0 PID: 5964 Comm: syz-executor151 Not tainted syzkaller #0 PREEMPT(full)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/07/2026
Call Trace:
<TASK>
dump_stack_lvl+0xe8/0x150 lib/dump_stack.c:120
print_circular_bug+0x2e2/0x300 kernel/locking/lockdep.c:2087
check_noncircular+0x12f/0x150 kernel/locking/lockdep.c:2219
check_prev_add kernel/locking/lockdep.c:3209 [inline]
check_prevs_add kernel/locking/lockdep.c:3328 [inline]
validate_chain kernel/locking/lockdep.c:3952 [inline]
__lock_acquire+0x164c/0x2de0 kernel/locking/lockdep.c:5288
lock_acquire+0x115/0x350 kernel/locking/lockdep.c:5942
d_wait_lookup fs/dcache.c:2840 [inline]
__d_alloc_parallel+0xc32/0x1700 fs/dcache.c:2944
__lookup_slow+0x82/0x2f0 fs/namei.c:1905
lookup_slow+0x59/0x80 fs/namei.c:1937
walk_component fs/namei.c:2316 [inline]
lookup_last fs/namei.c:2824 [inline]
path_lookupat+0x3f5/0x8c0 fs/namei.c:2857
filename_lookup+0x258/0x5f0 fs/namei.c:2886
do_readlinkat+0xe3/0x510 fs/stat.c:575
__do_sys_readlink fs/stat.c:609 [inline]
__se_sys_readlink fs/stat.c:606 [inline]
__x64_sys_readlink+0x7f/0x90 fs/stat.c:606
do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
do_syscall_64+0x166/0x520 arch/x86/entry/syscall_64.c:84
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f0e12a36e97
Code: 00 00 90 48 83 ec 10 48 63 ff 45 31 c9 45 31 c0 6a 00 31 c9 e8 4a 0a 02 00 48 83 c4 18 c3 0f 1f 44 00 00 b8 59 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 01 c3 48 c7 c2 d0 ff ff ff f7 d8 64 89 02 48
RSP: 002b:00007f0e12a02fe8 EFLAGS: 00000202 ORIG_RAX: 0000000000000059
RAX: ffffffffffffffda RBX: 0000000000000002 RCX: 00007f0e12a36e97
RDX: 0000000000000100 RSI: 00007f0e12a02ff0 RDI: 00007f0e12a030f0
RBP: 00007f0e12a030f0 R08: 00007f0e12ad9b20 R09: 0000000000000005
R10: 0000000000000000 R11: 0000000000000202 R12: 00007f0e12a02ff0
R13: 00007f0e12ad9b20 R14: 00007f0e12aa5028 R15: 00007ffda7595668
</TASK>
---
If you want syzbot to run the reproducer, reply with:
#syz test: git://repo/address.git branch-or-commit-hash
If you attach or paste a git patch, syzbot will apply it before testing.
prev parent reply other threads:[~2026-10-15 11:16 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-10-02 17:04 syzbot
2026-10-15 11:16 ` syzbot [this message]
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