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Linus Torvalds cfa2e34ffb rcu: locking and unlocking need to always be at least barriers 
commit 66be4e66a7 upstream.

Herbert Xu pointed out that commit bb73c52bad ("rcu: Don't disable
preemption for Tiny and Tree RCU readers") was incorrect in making the
preempt_disable/enable() be conditional on CONFIG_PREEMPT_COUNT.

If CONFIG_PREEMPT_COUNT isn't enabled, the preemption enable/disable is
a no-op, but still is a compiler barrier.

And RCU locking still _needs_ that compiler barrier.

It is simply fundamentally not true that RCU locking would be a complete
no-op: we still need to guarantee (for example) that things that can
trap and cause preemption cannot migrate into the RCU locked region.

The way we do that is by making it a barrier.

See for example commit 386afc9114 ("spinlocks and preemption points
need to be at least compiler barriers") from back in 2013 that had
similar issues with spinlocks that become no-ops on UP: they must still
constrain the compiler from moving other operations into the critical
region.

Now, it is true that a lot of RCU operations already use READ_ONCE() and
WRITE_ONCE() (which in practice likely would never be re-ordered wrt
anything remotely interesting), but it is also true that that is not
globally the case, and that it's not even necessarily always possible
(ie bitfields etc).

Reported-by: Herbert Xu <herbert@gondor.apana.org.au>
Fixes: bb73c52bad ("rcu: Don't disable preemption for Tiny and Tree RCU readers")
Cc: stable@kernel.org
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
2019-06-11 12:21:47 +02:00
arch Revert "x86/build: Move _etext to actual end of .text" 2019-06-09 09:18:20 +02:00
block block: sed-opal: fix IOC_OPAL_ENABLE_DISABLE_MBR 2019-05-31 06:47:29 -07:00
certs Replace magic for trusting the secondary keyring with #define 2018-09-09 19:55:54 +02:00
crypto crypto: ccm - fix incompatibility between "ccm" and "ccm_base" 2019-05-21 18:50:20 +02:00
Documentation doc: Cope with Sphinx logging deprecations 2019-06-09 09:18:17 +02:00
drivers net: sfp: read eeprom in maximum 16 byte increments 2019-06-11 12:21:46 +02:00
firmware License cleanup: add SPDX GPL-2.0 license identifier to files with no license 2017-11-02 11:10:55 +01:00
fs Revert "lockd: Show pid of lockd for remote locks" 2019-06-09 09:18:18 +02:00
include rcu: locking and unlocking need to always be at least barriers 2019-06-11 12:21:47 +02:00
init init: initialize jump labels before command line option parsing 2019-05-16 19:42:23 +02:00
ipc ipc/sem.c: prevent queue.status tearing in semop 2018-09-05 09:26:30 +02:00
kernel kernel/signal.c: trace_signal_deliver when signal_group_exit 2019-06-09 09:18:17 +02:00
lib kobject: Don't trigger kobject_uevent(KOBJ_REMOVE) twice. 2019-05-31 06:47:32 -07:00
mm memcg: make it work on sparse non-0-node systems 2019-06-09 09:18:17 +02:00
net Revert "fib_rules: return 0 directly if an exactly same rule exists when NLM_F_EXCL not supplied" 2019-06-11 12:21:47 +02:00
samples samples: mei: use /dev/mei0 instead of /dev/mei 2019-02-15 08:09:12 +01:00
scripts gcc-plugins: Fix build failures under Darwin host 2019-06-09 09:18:19 +02:00
security ima: show rules with IMA_INMASK correctly 2019-06-09 09:18:18 +02:00
sound ALSA: hda/realtek - Set default power save node to 0 2019-06-09 09:18:16 +02:00
tools libbpf: fix samples/bpf build failure due to undefined UINT32_MAX 2019-05-31 06:47:22 -07:00
usr initramfs: fix initramfs rebuilds w/ compression after disabling 2017-11-03 07:39:19 -07:00
virt KVM: s390: Do not report unusabled IDs via KVM_CAP_MAX_VCPU_ID 2019-06-09 09:18:16 +02:00
.cocciconfig
.get_maintainer.ignore
.gitattributes .gitattributes: set git diff driver for C source code files 2016-10-07 18:46:30 -07:00
.gitignore kbuild: rpm-pkg: keep spec file until make mrproper 2018-02-13 10:19:46 +01:00
.mailmap .mailmap: Add Maciej W. Rozycki's Imagination e-mail address 2017-11-10 12:16:15 -08:00
COPYING
CREDITS MAINTAINERS: update TPM driver infrastructure changes 2017-11-09 17:58:40 -08:00
Kbuild License cleanup: add SPDX GPL-2.0 license identifier to files with no license 2017-11-02 11:10:55 +01:00
Kconfig License cleanup: add SPDX GPL-2.0 license identifier to files with no license 2017-11-02 11:10:55 +01:00
MAINTAINERS MAINTAINERS: Add Sasha as a stable branch maintainer 2018-12-01 09:42:50 +01:00
Makefile Linux 4.14.124 2019-06-09 09:18:21 +02:00
README README: add a new README file, pointing to the Documentation/ 2016-10-24 08:12:35 -02:00

README 

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

This file was moved to Documentation/admin-guide/README.rst

Please notice that there are several guides for kernel developers and users.
These guides can be rendered in a number of formats, like HTML and PDF.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.

There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
See Documentation/00-INDEX for a list of what is contained in each file.

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.