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Qu Wenruo 108cc87339 btrfs: fix a lockdep caused by path resolution during device scan
[BUG]
There is a lockdep report related to device scan:

======================================================
WARNING: possible circular locking dependency detected
7.2.0-20260712.rc2.git0.e3321fa3034d.300.fc44.s390x+debug #1 Not tainted
------------------------------------------------------
(udev-worker)/1653 is trying to acquire lock:
0000006919232220 (&type->i_mutex_dir_key#2){++++}-{3:3}, at: lookup_slow+0x3e/0x70
but task is already holding lock:
00000069238564d8 (&fs_devs->device_list_mutex){+.+.}-{3:3}, at: device_list_add.constprop.0+0x148/0xc60
which lock already depends on the new lock.
the existing dependency chain (in reverse order) is:
-> #5 (&fs_devs->device_list_mutex){+.+.}-{3:3}:
       lock_acquire+0x150/0x3f0
       __mutex_lock+0xba/0xdc0
       mutex_lock_nested+0x32/0x40
       write_all_supers+0x7a/0x670
       btrfs_sync_log+0xae6/0xdd0
       btrfs_sync_file+0x4fa/0x7a0
       __s390x_sys_fsync+0x52/0xa0
       __do_syscall+0x172/0x750
       system_call+0x72/0x90
-> #4 (&fs_info->tree_log_mutex){+.+.}-{3:3}:
       lock_acquire+0x150/0x3f0
       __mutex_lock+0xba/0xdc0
       mutex_lock_nested+0x32/0x40
       btrfs_sync_log+0xaba/0xdd0
       btrfs_sync_file+0x4fa/0x7a0
       __s390x_sys_fsync+0x52/0xa0
       __do_syscall+0x172/0x750
       system_call+0x72/0x90
-> #3 (btrfs_trans_num_extwriters){.+.+}-{0:0}:
       lock_acquire+0x150/0x3f0
       join_transaction+0x108/0x680
       start_transaction+0x21a/0x660
       btrfs_join_transaction+0x32/0x40
       btrfs_dirty_inode+0x52/0xf0
       touch_atime+0x90/0xc0
       filemap_read+0x446/0x450
       vfs_read+0x208/0x370
       ksys_read+0x88/0x120
       __do_syscall+0x172/0x750
       system_call+0x72/0x90
-> #2 (btrfs_trans_num_writers){.+.+}-{0:0}:
       reacquire_held_locks+0x14c/0x240
       __lock_release.isra.0+0xd8/0x380
       lock_release+0xf6/0x270
       percpu_up_read+0x28/0xf0
       __btrfs_end_transaction+0x178/0x1f0
       btrfs_dirty_inode+0x82/0xf0
       touch_atime+0x90/0xc0
       btrfs_file_mmap_prepare+0x8c/0xa0
       __mmap_region+0x214/0x780
       mmap_region+0x108/0x160
       do_mmap+0x402/0x5a0
       vm_mmap_pgoff+0x156/0x230
       ksys_mmap_pgoff+0x17e/0x220
       __s390x_sys_old_mmap+0xa8/0x140
       __do_syscall+0x172/0x750
       system_call+0x72/0x90
-> #1 (&mm->mmap_lock){++++}-{3:3}:
       lock_acquire+0x150/0x3f0
       __might_fault+0x7a/0xa0
       filldir64+0x11c/0x210
       offset_readdir+0x92/0x200
       iterate_dir+0xcc/0x2d0
       __do_sys_getdents64+0x7a/0x130
       __do_syscall+0x172/0x750
       system_call+0x72/0x90
-> #0 (&type->i_mutex_dir_key#2){++++}-{3:3}:
       check_prev_add+0x160/0xf40
       __lock_acquire+0x12aa/0x15a0
       lock_acquire+0x150/0x3f0
       down_read+0x5a/0x280
       lookup_slow+0x3e/0x70
       path_lookupat+0x1f0/0x370
       filename_lookup+0xce/0x1f0
       kern_path+0x48/0x70
       is_same_device+0x146/0x300
       device_list_add.constprop.0+0x1be/0xc60
       btrfs_scan_one_device+0x13a/0x2f0
       btrfs_control_ioctl+0x110/0x1e0
       __s390x_sys_ioctl+0xfa/0x130
       __do_syscall+0x172/0x750
       system_call+0x72/0x90
other info that might help us debug this:
Chain exists of:
  &type->i_mutex_dir_key#2 --> &fs_info->tree_log_mutex --> &fs_devs->device_list_mutex
 Possible unsafe locking scenario:
       CPU0                    CPU1
       ----                    ----
  lock(&fs_devs->device_list_mutex);
                               lock(&fs_info->tree_log_mutex);
                               lock(&fs_devs->device_list_mutex);
  rlock(&type->i_mutex_dir_key#2);
 *** DEADLOCK ***
2 locks held by (udev-worker)/1653:
 #0: 0000016c727051c8 (uuid_mutex){+.+.}-{3:3}, at: btrfs_control_ioctl+0x102/0x1e0
 #1: 00000069238564d8 (&fs_devs->device_list_mutex){+.+.}-{3:3}, at: device_list_add.constprop.0+0x148/0xc60
stack backtrace:
CPU: 2 UID: 0 PID: 1653 Comm: (udev-worker) Not tainted 7.2.0-20260712.rc2.git0.e3321fa3034d.300.fc44.s390x+debug #1 PREEMPT
Hardware name: IBM 3931 A01 701 (LPAR)
Call Trace:
 [<0000016c70680e3e>] dump_stack_lvl+0xae/0x108
 [<0000016c7078aa44>] print_circular_bug+0x1a4/0x230
 [<0000016c7078ac5c>] check_noncircular+0x18c/0x1b0
 [<0000016c7078c030>] check_prev_add+0x160/0xf40
 [<0000016c7078fbaa>] __lock_acquire+0x12aa/0x15a0
 [<0000016c7078fff0>] lock_acquire+0x150/0x3f0
 [<0000016c7180e2fa>] down_read+0x5a/0x280
 [<0000016c70b88dde>] lookup_slow+0x3e/0x70
 [<0000016c70b8f5d0>] path_lookupat+0x1f0/0x370
 [<0000016c70b900ae>] filename_lookup+0xce/0x1f0
 [<0000016c70b90218>] kern_path+0x48/0x70
 [<0000016c70f5b4a6>] is_same_device+0x146/0x300
 [<0000016c70f67cfe>] device_list_add.constprop.0+0x1be/0xc60
 [<0000016c70f688da>] btrfs_scan_one_device+0x13a/0x2f0
 [<0000016c70ed9cb0>] btrfs_control_ioctl+0x110/0x1e0
 [<0000016c70b97d0a>] __s390x_sys_ioctl+0xfa/0x130
 [<0000016c718004d2>] __do_syscall+0x172/0x750
 [<0000016c718155d2>] system_call+0x72/0x90

[CAUSE]
Btrfs device scan will call is_same_device() with device_list_mutex
held. But is_same_device() will call kern_path() which will do path
resolution and lock the inode.

So device scan has the following lock sequence:

 mutex_lock(device_list_mutex) from device_list_add()
   |
   v
 inode_lock_shared() from lookup_slow() during kern_path().

Meanwhile another thread is fsyncing, which has the following
lock sequence:

 inode_lock() from btrfs_inode_lock() inside btrfs_direct_write()
   |
   v
 mutex_lock(tree_log_mutex() from btrfs_sync_log(), which is further
 triggered from
 iomap_dio_complete()->generic_write_sync()->btrfs_sync_file().
   |
   v
 mutex_lock(device_list_mutex) from write_all_supers() inside
 btrfs_sync_log().

So the device scan has a reversed lock sequence, compared to the fsync
one, this means we can have the following deadlock:

          Device scan                   |           Fsync
----------------------------------------+--------------------------------
 device_list_mutex locked               |
                                        |   inode locked
					|   try to lock device_list_mutex
 try to lock inode                      |

[FIX]
Instead of a full path lookup, use dev_t to determine if two device
paths are pointing to the same block device.

Inside kernel dev_t is going to uniquely determine a block device, and
the device path lookup is already done by lookup_bdev(), which is done
without device_list_mutex held, thus no reversed locking sequence.

Reported-by: Christian Borntraeger <borntraeger@linux.ibm.com>
Link: https://lore.kernel.org/linux-btrfs/5a9d9847-4ae6-43c4-afdc-6e5fa51d6117@linux.ibm.com/
Fixes: 2e8b6bc0ab ("btrfs: avoid unnecessary device path update for the same device")
Tested-by: Christian Borntraeger <borntraeger@linux.ibm.com>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
History 2026-08-07 19:17:17 +02:00
arch s390: 2026-08-07 07:41:40 -07:00
block block: stop the timeout timer when releasing a never added disk 2026-07-29 05:23:20 -06:00
certs treewide: fix indentation and whitespace in Kconfig files 2026-05-28 21:24:42 -07:00
crypto crypto: aes - Fix conditions for selecting MAC dependencies 2026-07-09 11:06:22 -04:00
Documentation Including fixes from netfilter. 2026-08-06 11:39:20 -07:00
drivers s390: 2026-08-07 07:41:40 -07:00
fs btrfs: fix a lockdep caused by path resolution during device scan 2026-08-07 19:17:17 +02:00
include Thermal control fixes for 7.2-rc7 2026-08-07 06:48:51 -07:00
init rust: allow suspicious_runtime_symbol_definitions lint for Rust >= 1.98 2026-06-29 13:02:15 +02:00
io_uring io_uring: preserve task restrictions across exec 2026-07-30 14:40:22 -06:00
ipc kernel-7.2-rc1.misc 2026-06-15 03:05:50 +05:30
kernel BPF fixes: 2026-08-07 08:08:57 -07:00
lib hardening fixes for v7.2-rc6 2026-07-28 13:36:01 -07:00
LICENSES LICENSES: Add modern form of the LGPL-2.1 tags to the usage guide section 2025-10-22 07:58:19 +02:00
mm 17 hotfixes. 15 are cc:stable. 16 are for MM. 2026-08-06 20:29:38 -07:00
net BPF fixes: 2026-08-07 08:08:57 -07:00
rust rust: time: fix as_micros_ceil() to round correctly for negative Delta 2026-07-18 19:07:35 +02:00
samples samples/damon/mtier: error out for zero quota goal target values 2026-08-04 20:02:02 -07:00
scripts kbuild: rpm-pkg: Preserve BTF sections in kernel modules during debuginfo stripping 2026-07-29 16:21:42 -07:00
security selinux/stable-7.2 PR 20260805 2026-08-05 13:40:11 -07:00
sound ALSA: usb-audio: Fix sticky mixer regressions on M-Audio Fast Track Ultra 2026-08-07 10:35:31 +02:00
tools BPF fixes: 2026-08-07 08:08:57 -07:00
usr kbuild: uapi: also test UAPI headers against C++ compilers 2026-03-25 13:24:42 +01:00
virt KVM/arm64 fixes for 7.2, take #2 2026-07-15 12:13:30 +02:00
.clang-format clang-format: fix formatting of guard() and scoped_guard() statements 2026-05-28 21:24:41 -07:00
.clippy.toml rust: bump Clippy's MSRV and clean incompatible_msrv allows 2026-04-07 09:51:39 +02:00
.cocciconfig scripts: add Linux .cocciconfig for coccinelle 2016-07-22 12:13:39 +02:00
.editorconfig editorconfig: add rst extension 2026-01-26 19:07:09 -08:00
.get_maintainer.ignore .get_maintainer.ignore: add myself 2026-04-02 16:48:25 +02:00
.gitattributes .gitattributes: set diff driver for Rust source code files 2023-05-31 17:48:25 +02:00
.gitignore SPDX patches for 7.2-rc1 2026-06-22 12:06:22 -07:00
.mailmap 17 hotfixes. 15 are cc:stable. 16 are for MM. 2026-08-06 20:29:38 -07:00
.pylintrc docs: Move the python libraries to tools/lib/python 2025-11-18 09:22:40 -07:00
.rustfmt.toml rust: add .rustfmt.toml 2022-09-28 09:02:20 +02:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS CREDITS: Add Wolfram Sang 2026-07-14 16:34:04 +02:00
Kbuild checksyscalls: move instance functionality into generic code 2026-04-05 09:21:32 +02:00
Kconfig io_uring: Rename KConfig to Kconfig 2025-02-19 14:53:27 -07:00
MAINTAINERS sound fixes for 7.2-rc7 2026-08-07 06:36:11 -07:00
Makefile Linux 7.2-rc6 2026-08-02 16:24:24 -07:00
README docs: add AI Coding Assistants documentation 2026-01-06 14:55:06 -07:00

Linux kernel
============

The Linux kernel is the core of any Linux operating system. It manages hardware,
system resources, and provides the fundamental services for all other software.

Quick Start
-----------

* Report a bug: See Documentation/admin-guide/reporting-issues.rst
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-----------------------

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

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

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--------------------

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-------------------

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---------------

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-----------------------------

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--------------------

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----------

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---------------

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-----------------------

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-------------------

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

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