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Christian Brauner 749d7aa037
super: fix emergency thaw deadlock on frozen block devices
do_thaw_all_callback() calls bdev_thaw() while holding sb->s_umount
exclusively. If the block device was frozen via bdev_freeze() dropping
the last block layer freeze reference calls fs_bdev_thaw() which
reacquires s_umount:

  do_thaw_all_callback(sb)
    super_lock_excl(sb)                     # holds sb->s_umount
    bdev_thaw(sb->s_bdev)
      mutex_lock(&bdev->bd_fsfreeze_mutex)
      # bd_fsfreeze_count drops 1 -> 0
      bd_holder_ops->thaw == fs_bdev_thaw
        get_bdev_super(bdev)
          bdev_super_lock(bdev, true)
            super_lock(sb, true)
              down_write(&sb->s_umount)     # same task: deadlock

The emergency thaw worker deadlocks against itself holding both
s_umount and bd_fsfreeze_mutex. That fscks any subsequent unmount,
freeze, or thaw of that filesystem and block device.

  [   81.878470] sysrq: Show Blocked State
  [   81.880140] task:kworker/0:1     state:D stack:0     pid:11    tgid:11    ppid:2      task_flags:0x4208060 flags:0x00080000
  [   81.884876] Workqueue: events do_thaw_all
  [   81.886656] Call Trace:
  [   81.887759]  <TASK>
  [   81.888763]  __schedule+0x579/0x1420
  [   81.890372]  schedule+0x3a/0x100
  [   81.891794]  schedule_preempt_disabled+0x15/0x30
  [   81.893848]  rwsem_down_write_slowpath+0x1ea/0x900
  [   81.895191]  ? __pfx_do_thaw_all_callback+0x10/0x10
  [   81.896528]  down_write+0xbd/0xc0
  [   81.897505]  super_lock+0x91/0x180
  [   81.898457]  ? __mutex_lock+0xa99/0x1140
  [   81.900748]  ? __mutex_unlock_slowpath+0x1f/0x400
  [   81.902069]  bdev_super_lock+0x5b/0x150
  [   81.903132]  get_bdev_super+0x10/0x60
  [   81.904042]  fs_bdev_thaw+0x23/0xf0
  [   81.904755]  bdev_thaw+0x82/0x100
  [   81.905484]  do_thaw_all_callback+0x2c/0x50
  [   81.906298]  __iterate_supers+0x5d/0x130
  [   81.907067]  do_thaw_all+0x20/0x40
  [   81.907739]  process_one_work+0x206/0x5e0
  [   81.908545]  worker_thread+0x1e2/0x3c0
  [   81.909339]  ? __pfx_worker_thread+0x10/0x10
  [   81.910171]  kthread+0xf4/0x130
  [   81.910799]  ? __pfx_kthread+0x10/0x10
  [   81.911528]  ret_from_fork+0x2e2/0x3b0
  [   81.912259]  ? __pfx_kthread+0x10/0x10
  [   81.913010]  ret_from_fork_asm+0x1a/0x30
  [   81.913806]  </TASK>

bdev_super_lock() even documents the violated requirement with
lockdep_assert_not_held(&sb->s_umount).

Acquiring bd_fsfreeze_mutex under s_umount also inverts the
bd_fsfreeze_mutex vs. s_umount ordering established by
bdev_{freeze,thaw}() and can thus ABBA against a concurrent block-layer
freeze even when the recursive path isn't hit.

Fix this by not holding s_umount around the bdev_thaw() loop at all. Pin
the superblock with an active reference instead as
filesystems_freeze_callback() does. The active reference keeps the
superblock from being shut down and so ->s_bdev stays valid without
holding s_umount. The block-layer-held freeze is dropped by
fs_bdev_thaw() with FREEZE_MAY_NEST | FREEZE_HOLDER_USERSPACE exactly as
a regular unfreeze would and thaw_super_locked() handles
filesystem-level freezes as before.

The emergency thaw path has deadlocked like this in one form or
another for a long long time but the current exclusively-held
shape dates back to commit [1] where thaw_bdev() already ended in
thaw_super() with s_umount held by do_thaw_all_callback().

Fixes: 08fdc8a013 ("buffer.c: call thaw_super during emergency thaw") [1]
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20260723-work-super-emergency_thaw-v1-1-7c315c600245@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
2026-07-26 17:08:52 +02:00
arch RISC-V updates for v7.2-rc4 2026-07-19 12:41:00 -07:00
block block: fix aligning of bounced dio read bios 2026-07-16 06:11:37 -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 Crypto library fixes for v7.2-rc4 2026-07-17 10:53:57 -07:00
drivers block-7.2-20260717 2026-07-19 09:29:42 -07:00
fs super: fix emergency thaw deadlock on frozen block devices 2026-07-26 17:08:52 +02:00
include fs: push nr_cached_objects memcg gating into individual filesystems 2026-07-23 11:35:02 +02:00
init mm.git review status for mm-hotfixes-stable..mm-nonmm-stable 2026-06-21 13:20:19 -07:00
io_uring io_uring-7.2-20260717 2026-07-19 09:24:32 -07:00
ipc kernel-7.2-rc1.misc 2026-06-15 03:05:50 +05:30
kernel BPF fixes: 2026-07-17 12:55:24 -07:00
lib Crypto library fixes for v7.2-rc4 2026-07-17 10:53:57 -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 fs: push nr_cached_objects memcg gating into individual filesystems 2026-07-23 11:35:02 +02:00
net BPF fixes: 2026-07-17 12:55:24 -07:00
rust Rust changes for v7.2 (addendum) 2026-06-24 15:11:08 -07:00
samples tracing fixes for 7.2: 2026-07-12 09:46:37 -07:00
scripts scripts/sorttable: Handle RISC-V patchable ftrace entries 2026-06-25 13:15:03 -06:00
security selinux/stable-7.2 PR 20260717 2026-07-17 11:04:46 -07:00
sound ALSA: usb-audio: Add delay quirk for iBasso DC-Elite 2026-07-13 17:06:20 +02:00
tools selftests/pidfd: check PIDFD_THREAD survives open_by_handle_at() 2026-07-22 13:55:20 +02:00
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
.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 Including fixes from Wireless, IPsec, Netfilter and Bluetooth. 2026-07-17 10:25:13 -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
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 SoC fixes for 7.2 2026-07-17 08:52:09 -07:00
Makefile Linux 7.2-rc4 2026-07-19 13:54:41 -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
* Get the latest kernel: https://kernel.org
* Build the kernel: See Documentation/admin-guide/quickly-build-trimmed-linux.rst
* Join the community: https://lore.kernel.org/

Essential Documentation
-----------------------

All users should be familiar with:

* Building requirements: Documentation/process/changes.rst
* Code of Conduct: Documentation/process/code-of-conduct.rst
* License: See COPYING

Documentation can be built with make htmldocs or viewed online at:
https://www.kernel.org/doc/html/latest/


Who Are You?
============

Find your role below:

* New Kernel Developer - Getting started with kernel development
* Academic Researcher - Studying kernel internals and architecture
* Security Expert - Hardening and vulnerability analysis
* Backport/Maintenance Engineer - Maintaining stable kernels
* System Administrator - Configuring and troubleshooting
* Maintainer - Leading subsystems and reviewing patches
* Hardware Vendor - Writing drivers for new hardware
* Distribution Maintainer - Packaging kernels for distros
* AI Coding Assistant - LLMs and AI-powered development tools


For Specific Users
==================

New Kernel Developer
--------------------

Welcome! Start your kernel development journey here:

* Getting Started: Documentation/process/development-process.rst
* Your First Patch: Documentation/process/submitting-patches.rst
* Coding Style: Documentation/process/coding-style.rst
* Build System: Documentation/kbuild/index.rst
* Development Tools: Documentation/dev-tools/index.rst
* Kernel Hacking Guide: Documentation/kernel-hacking/hacking.rst
* Core APIs: Documentation/core-api/index.rst

Academic Researcher
-------------------

Explore the kernel's architecture and internals:

* Researcher Guidelines: Documentation/process/researcher-guidelines.rst
* Memory Management: Documentation/mm/index.rst
* Scheduler: Documentation/scheduler/index.rst
* Networking Stack: Documentation/networking/index.rst
* Filesystems: Documentation/filesystems/index.rst
* RCU (Read-Copy Update): Documentation/RCU/index.rst
* Locking Primitives: Documentation/locking/index.rst
* Power Management: Documentation/power/index.rst

Security Expert
---------------

Security documentation and hardening guides:

* Security Documentation: Documentation/security/index.rst
* LSM Development: Documentation/security/lsm-development.rst
* Self Protection: Documentation/security/self-protection.rst
* Reporting Vulnerabilities: Documentation/process/security-bugs.rst
* CVE Procedures: Documentation/process/cve.rst
* Embargoed Hardware Issues: Documentation/process/embargoed-hardware-issues.rst
* Security Features: Documentation/userspace-api/seccomp_filter.rst

Backport/Maintenance Engineer
-----------------------------

Maintain and stabilize kernel versions:

* Stable Kernel Rules: Documentation/process/stable-kernel-rules.rst
* Backporting Guide: Documentation/process/backporting.rst
* Applying Patches: Documentation/process/applying-patches.rst
* Subsystem Profile: Documentation/maintainer/maintainer-entry-profile.rst
* Git for Maintainers: Documentation/maintainer/configure-git.rst

System Administrator
--------------------

Configure, tune, and troubleshoot Linux systems:

* Admin Guide: Documentation/admin-guide/index.rst
* Kernel Parameters: Documentation/admin-guide/kernel-parameters.rst
* Sysctl Tuning: Documentation/admin-guide/sysctl/index.rst
* Tracing/Debugging: Documentation/trace/index.rst
* Performance Security: Documentation/admin-guide/perf-security.rst
* Hardware Monitoring: Documentation/hwmon/index.rst

Maintainer
----------

Lead kernel subsystems and manage contributions:

* Maintainer Handbook: Documentation/maintainer/index.rst
* Pull Requests: Documentation/maintainer/pull-requests.rst
* Managing Patches: Documentation/maintainer/modifying-patches.rst
* Rebasing and Merging: Documentation/maintainer/rebasing-and-merging.rst
* Development Process: Documentation/process/maintainer-handbooks.rst
* Maintainer Entry Profile: Documentation/maintainer/maintainer-entry-profile.rst
* Git Configuration: Documentation/maintainer/configure-git.rst

Hardware Vendor
---------------

Write drivers and support new hardware:

* Driver API Guide: Documentation/driver-api/index.rst
* Driver Model: Documentation/driver-api/driver-model/driver.rst
* Device Drivers: Documentation/driver-api/infrastructure.rst
* Bus Types: Documentation/driver-api/driver-model/bus.rst
* Device Tree Bindings: Documentation/devicetree/bindings/
* Power Management: Documentation/driver-api/pm/index.rst
* DMA API: Documentation/core-api/dma-api.rst

Distribution Maintainer
-----------------------

Package and distribute the kernel:

* Stable Kernel Rules: Documentation/process/stable-kernel-rules.rst
* ABI Documentation: Documentation/ABI/README
* Kernel Configuration: Documentation/kbuild/kconfig.rst
* Module Signing: Documentation/admin-guide/module-signing.rst
* Kernel Parameters: Documentation/admin-guide/kernel-parameters.rst
* Tainted Kernels: Documentation/admin-guide/tainted-kernels.rst

AI Coding Assistant
-------------------

CRITICAL: If you are an LLM or AI-powered coding assistant, you MUST read and
follow the AI coding assistants documentation before contributing to the Linux
kernel:

* Documentation/process/coding-assistants.rst

This documentation contains essential requirements about licensing, attribution,
and the Developer Certificate of Origin that all AI tools must comply with.


Communication and Support
=========================

* Mailing Lists: https://lore.kernel.org/
* IRC: #kernelnewbies on irc.oftc.net
* Bugzilla: https://bugzilla.kernel.org/
* MAINTAINERS file: Lists subsystem maintainers and mailing lists
* Email Clients: Documentation/process/email-clients.rst