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Frederic Weisbecker 868c9037df timers/migration: Fix off-by-one root mis-connection 
Before attaching a new root to the old root, the children counter of the
new root is checked to verify that only the upcoming CPU's top group have
been connected to it. However since the recently added commit b729cc1ec2
("timers/migration: Fix another race between hotplug and idle entry/exit")
this check is not valid anymore because the old root is pre-accounted
as a child to the new root. Therefore after connecting the upcoming
CPU's top group to the new root, the children count to be expected must
be 2 and not 1 anymore.

This omission results in the old root to not be connected to the new
root. Then eventually the system may run with more than one top level,
which defeats the purpose of a single idle migrator.

Also the old root is pre-accounted but not connected upon the new root
creation. But it can be connected to the new root later on. Therefore
the old root may be accounted twice to the new root. The propagation of
such overcommit can end up creating a double final top-level root with a
groupmask incorrectly initialized. Although harmless given that the final
top level roots will never have a parent to walk up to, this oddity
opportunistically reported the core issue:

  WARNING: CPU: 8 PID: 0 at kernel/time/timer_migration.c:543 tmigr_requires_handle_remote
  CPU: 8 UID: 0 PID: 0 Comm: swapper/8
  RIP: 0010:tmigr_requires_handle_remote
  Call Trace:
   <IRQ>
   ? tmigr_requires_handle_remote
   ? hrtimer_run_queues
   update_process_times
   tick_periodic
   tick_handle_periodic
   __sysvec_apic_timer_interrupt
   sysvec_apic_timer_interrupt
  </IRQ>

Fix the problem by taking the old root into account in the children count
of the new root so the connection is not omitted.

Also warn when more than one top level group exists to better detect
similar issues in the future.

Fixes: b729cc1ec2 ("timers/migration: Fix another race between hotplug and idle entry/exit")
Reported-by: Matt Fleming <mfleming@cloudflare.com>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/all/20250205160220.39467-1-frederic@kernel.org
2025-02-07 09:02:16 +01:00
arch Kthreads affinity follow either of 4 existing different patterns: 2025-01-21 17:10:05 -08:00
block for-6.14/io_uring-20250119 2025-01-20 20:27:33 -08:00
certs sign-file,extract-cert: use pkcs11 provider for OPENSSL MAJOR >= 3 2024-09-20 19:52:48 +03:00
crypto Kthreads affinity follow either of 4 existing different patterns: 2025-01-21 17:10:05 -08:00
Documentation chrome-platform: Updates for v6.14 2025-01-21 19:48:29 -08:00
drivers lsm/stable-6.14 PR 20250121 2025-01-21 20:03:04 -08:00
fs lsm/stable-6.14 PR 20250121 2025-01-21 20:03:04 -08:00
include jiffies: Cast to unsigned long in secs_to_jiffies() conversion 2025-01-31 09:30:49 +01:00
init Rust changes for v6.14 2025-01-21 17:48:03 -08:00
io_uring for-6.14/io_uring-20250119 2025-01-20 20:27:33 -08:00
ipc - The series "resource: A couple of cleanups" from Andy Shevchenko 2024-11-25 16:09:48 -08:00
kernel timers/migration: Fix off-by-one root mis-connection 2025-02-07 09:02:16 +01:00
lib Documentation changes this time around include: 2025-01-21 18:00:00 -08:00
LICENSES LICENSES: add 0BSD license text 2024-09-01 20:43:24 -07:00
mm Kthreads affinity follow either of 4 existing different patterns: 2025-01-21 17:10:05 -08:00
net lsm/stable-6.14 PR 20250121 2025-01-21 20:03:04 -08:00
rust lsm/stable-6.14 PR 20250121 2025-01-21 20:03:04 -08:00
samples Rust changes for v6.14 2025-01-21 17:48:03 -08:00
scripts Documentation changes this time around include: 2025-01-21 18:00:00 -08:00
security selinux/stable-6.14 PR 20250121 2025-01-21 20:09:14 -08:00
sound drm next for 6.14-rc1 2025-01-21 16:09:47 -08:00
tools lsm/stable-6.14 PR 20250121 2025-01-21 20:03:04 -08:00
usr kbuild: Drop support for include/asm-<arch> in headers_check.pl 2024-12-21 11:43:17 +09:00
virt VFIO updates for v6.13 2024-11-27 12:57:03 -08:00
.clang-format clang-format: Update with v6.11-rc1's for_each macro list 2024-08-02 13:20:31 +02:00
.clippy.toml rust: give Clippy the minimum supported Rust version 2025-01-10 00:17:25 +01:00
.cocciconfig scripts: add Linux .cocciconfig for coccinelle 2016-07-22 12:13:39 +02:00
.editorconfig .editorconfig: remove trim_trailing_whitespace option 2024-06-13 16:47:52 +02:00
.get_maintainer.ignore MAINTAINERS: Retire Ralf Baechle 2024-11-12 15:48:59 +01:00
.gitattributes .gitattributes: set diff driver for Rust source code files 2023-05-31 17:48:25 +02:00
.gitignore rust: use host dylib naming convention to support macOS 2025-01-10 01:01:24 +01:00
.mailmap 7 singleton hotfixes. 6 are MM. 2 are cc:stable and the remainder 2025-01-16 21:24:34 -08:00
.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 MAINTAINERS: remove Andy Gospodarek from bonding 2025-01-09 08:30:01 -08:00
Kbuild Kbuild updates for v6.1 2022-10-10 12:00:45 -07:00
Kconfig kbuild: ensure full rebuild when the compiler is updated 2020-05-12 13:28:33 +09:00
MAINTAINERS Documentation changes this time around include: 2025-01-21 18:00:00 -08:00
Makefile Rust changes for v6.14 2025-01-21 17:48:03 -08:00
README README: Fix spelling 2024-03-18 03:36:32 -06:00

README 

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

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.