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Frederic Weisbecker 34255103ff rcu/nocb: Remove buggy bypass lock contention mitigation 
[ Upstream commit e4f7805729 ]

The bypass lock contention mitigation assumes there can be at most
2 contenders on the bypass lock, following this scheme:

1) One kthread takes the bypass lock
2) Another one spins on it and increment the contended counter
3) A third one (a bypass enqueuer) sees the contended counter on and
  busy loops waiting on it to decrement.

However this assumption is wrong. There can be only one CPU to find the
lock contended because call_rcu() (the bypass enqueuer) is the only
bypass lock acquire site that may not already hold the NOCB lock
beforehand, all the other sites must first contend on the NOCB lock.
Therefore step 2) is impossible.

The other problem is that the mitigation assumes that contenders all
belong to the same rdp CPU, which is also impossible for a raw spinlock.
In theory the warning could trigger if the enqueuer holds the bypass
lock and another CPU flushes the bypass queue concurrently but this is
prevented from all flush users:

1) NOCB kthreads only flush if they successfully _tried_ to lock the
   bypass lock. So no contention management here.

2) Flush on callbacks migration happen remotely when the CPU is offline.
   No concurrency against bypass enqueue.

3) Flush on deoffloading happen either locally with IRQs disabled or
   remotely when the CPU is not yet online. No concurrency against
   bypass enqueue.

4) Flush on barrier entrain happen either locally with IRQs disabled or
   remotely when the CPU is offline. No concurrency against
   bypass enqueue.

For those reasons, the bypass lock contention mitigation isn't needed
and is even wrong. Remove it but keep the warning reporting a contended
bypass lock on a remote CPU, to keep unexpected contention awareness.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
History 2024-09-12 11:07:41 +02:00
arch Revert "MIPS: Loongson64: reset: Prioritise firmware service" 2024-09-04 13:23:38 +02:00
block block: initialize integrity buffer to zero before writing it to media 2024-08-19 05:44:50 +02:00
certs certs/blacklist_hashes.c: fix const confusion in certs blacklist 2022-06-22 14:22:01 +02:00
crypto crypto: aead,cipher - zeroize key buffer after use 2024-07-18 13:07:27 +02:00
Documentation hwspinlock: Introduce hwspin_lock_bust() 2024-09-12 11:07:41 +02:00
drivers ionic: fix potential irq name truncation 2024-09-12 11:07:41 +02:00
fs btrfs: run delayed iputs when flushing delalloc 2024-09-04 13:23:37 +02:00
include hwspinlock: Introduce hwspin_lock_bust() 2024-09-12 11:07:41 +02:00
init init/main.c: Fix potential static_command_line memory overflow 2024-04-27 17:05:28 +02:00
io_uring io_uring/io-wq: limit retrying worker initialisation 2024-08-19 05:45:22 +02:00
ipc ipc/sem: Fix dangling sem_array access in semtimedop race 2022-12-08 11:28:45 +01:00
kernel rcu/nocb: Remove buggy bypass lock contention mitigation 2024-09-12 11:07:41 +02:00
lib bitmap: introduce generic optimized bitmap_size() 2024-09-04 13:23:16 +02:00
LICENSES LICENSES/dual/CC-BY-4.0: Git rid of "smart quotes" 2021-07-15 06:31:24 -06:00
mm mm/numa: no task_numa_fault() call if PTE is changed 2024-09-04 13:23:37 +02:00
net ethtool: check device is present when getting link settings 2024-09-04 13:23:40 +02:00
samples Add gitignore file for samples/fanotify/ subdirectory 2024-07-27 10:46:16 +02:00
scripts kbuild: Fix '-S -c' in x86 stack protector scripts 2024-08-19 05:45:16 +02:00
security apparmor: fix possible NULL pointer dereference 2024-09-12 11:07:40 +02:00
sound ALSA: hda/conexant: Mute speakers at suspend / shutdown 2024-09-12 11:07:38 +02:00
tools tools: move alignment-related macros to new <linux/align.h> 2024-09-04 13:23:37 +02:00
usr usr/include/Makefile: add linux/nfc.h to the compile-test coverage 2022-02-01 17:27:15 +01:00
virt KVM: Always flush async #PF workqueue when vCPU is being destroyed 2024-04-10 16:18:34 +02:00
.clang-format clang-format: Update with the latest for_each macro list 2021-05-12 23:32:39 +02:00
.cocciconfig scripts: add Linux .cocciconfig for coccinelle 2016-07-22 12:13:39 +02:00
.get_maintainer.ignore Opt out of scripts/get_maintainer.pl 2019-05-16 10:53:40 -07:00
.gitattributes .gitattributes: use 'dts' diff driver for dts files 2019-12-04 19:44:11 -08:00
.gitignore .gitignore: ignore only top-level modules.builtin 2021-05-02 00:43:35 +09:00
.mailmap mailmap: add Andrej Shadura 2021-10-18 20:22:03 -10:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS MAINTAINERS: Move Daniel Drake to credits 2021-09-21 08:34:58 +03:00
Kbuild kbuild: rename hostprogs-y/always to hostprogs/always-y 2020-02-04 01:53:07 +09:00
Kconfig kbuild: ensure full rebuild when the compiler is updated 2020-05-12 13:28:33 +09:00
MAINTAINERS trace: Relocate event helper files 2024-04-10 16:19:24 +02:00
Makefile Linux 5.15.166 2024-09-04 13:23:42 +02:00
README Drop all 00-INDEX files from Documentation/ 2018-09-09 15:08:58 -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 Restructured Text 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.