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Vlastimil Babka 59450bbc12 mm, slab, slub: stop taking cpu hotplug lock 
SLAB has been using get/put_online_cpus() around creating, destroying and
shrinking kmem caches since 95402b3829 ("cpu-hotplug: replace
per-subsystem mutexes with get_online_cpus()") in 2008, which is supposed
to be replacing a private mutex (cache_chain_mutex, called slab_mutex
today) with system-wide mechanism, but in case of SLAB it's in fact used
in addition to the existing mutex, without explanation why.

SLUB appears to have avoided the cpu hotplug lock initially, but gained it
due to common code unification, such as 20cea9683e ("mm, sl[aou]b: Move
kmem_cache_create mutex handling to common code").

Regardless of the history, checking if the hotplug lock is actually needed
today suggests that it's not, and therefore it's better to avoid this
system-wide lock and the ordering this imposes wrt other locks (such as
slab_mutex).

Specifically, in SLAB we have for_each_online_cpu() in do_tune_cpucache()
protected by slab_mutex, and cpu hotplug callbacks that also take the
slab_mutex, which is also taken by the common slab function that currently
also take the hotplug lock.  Thus the slab_mutex protection should be
sufficient.  Also per-cpu array caches are allocated for each possible
cpu, so not affected by their online/offline state.

In SLUB we have for_each_online_cpu() in functions that show statistics
and are already unprotected today, as racing with hotplug is not harmful.
Otherwise SLUB relies on percpu allocator.  The slub_cpu_dead() hotplug
callback takes the slab_mutex.

To sum up, this patch removes get/put_online_cpus() calls from slab as it
should be safe without further adjustments.

Link: https://lkml.kernel.org/r/20210113131634.3671-4-vbabka@suse.cz
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Christoph Lameter <cl@linux.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Qian Cai <cai@redhat.com>
Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2021-02-24 13:38:27 -08:00
arch hexagon: remove CONFIG_EXPERIMENTAL from defconfigs 2021-02-24 13:38:26 -08:00
block for-5.12/block-ipi-2021-02-21 2021-02-22 10:53:05 -08:00
certs certs: Replace K{U,G}IDT_INIT() with GLOBAL_ROOT_{U,G}ID 2021-01-21 16:16:10 +00:00
crypto Keyrings miscellany 2021-02-23 16:09:23 -08:00
Documentation Keyrings miscellany 2021-02-23 16:09:23 -08:00
drivers Merge git://git.kernel.org/pub/scm/linux/kernel/git/davem/sparc 2021-02-23 15:09:53 -08:00
fs ramfs: support O_TMPFILE 2021-02-24 13:38:26 -08:00
include mm, tracing: record slab name for kmem_cache_free() 2021-02-24 13:38:26 -08:00
init Kbuild: disable TRIM_UNUSED_KSYMS option 2021-02-23 12:21:58 -08:00
ipc fs: make helpers idmap mount aware 2021-01-24 14:27:20 +01:00
kernel Keyrings miscellany 2021-02-23 16:09:23 -08:00
lib Modules updates for v5.12 2021-02-23 10:15:33 -08:00
LICENSES LICENSES: Add the CC-BY-4.0 license 2020-12-08 10:33:27 -07:00
mm mm, slab, slub: stop taking cpu hotplug lock 2021-02-24 13:38:27 -08:00
net idmapped-mounts-v5.12 2021-02-23 13:39:45 -08:00
samples Keyrings miscellany 2021-02-23 16:09:23 -08:00
scripts scripts/spelling.txt: add more spellings to spelling.txt 2021-02-24 13:38:26 -08:00
security Keyrings miscellany 2021-02-23 16:09:23 -08:00
sound ARM updates for 5.12-rc1: 2021-02-22 14:27:07 -08:00
tools clang-lto for v5.12-rc1 (part2) 2021-02-23 15:13:45 -08:00
usr arch: ia64: Remove rest of perfmon support 2021-01-22 12:12:20 +05:30
virt KVM/arm64 fixes for 5.11, take #2 2021-02-12 14:07:39 +00:00
.clang-format clang-format: Update with the latest for_each macro list 2021-01-29 15:00:23 +01:00
.cocciconfig
.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 clang-lto series for v5.12-rc1 2021-02-23 09:28:51 -08:00
.mailmap Merge branch 'linus' of git://git.kernel.org/pub/scm/linux/kernel/git/herbert/crypto-2.6 2021-02-21 17:23:56 -08:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS MAINTAINERS: dccp: move Gerrit Renker to CREDITS 2021-01-14 10:53:49 -08:00
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 dmaengine updates for v5.12-rc1 2021-02-23 15:05:10 -08:00
Makefile clang-lto for v5.12-rc1 (part2) 2021-02-23 15:13:45 -08:00
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.