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Waiman Long 76723ed1fb locking/rwsem: Make handoff bit handling more consistent [ Upstream commit `d257cc8cb8` ] There are some inconsistency in the way that the handoff bit is being handled in readers and writers that lead to a race condition. Firstly, when a queue head writer set the handoff bit, it will clear it when the writer is being killed or interrupted on its way out without acquiring the lock. That is not the case for a queue head reader. The handoff bit will simply be inherited by the next waiter. Secondly, in the out_nolock path of rwsem_down_read_slowpath(), both the waiter and handoff bits are cleared if the wait queue becomes empty. For rwsem_down_write_slowpath(), however, the handoff bit is not checked and cleared if the wait queue is empty. This can potentially make the handoff bit set with empty wait queue. Worse, the situation in rwsem_down_write_slowpath() relies on wstate, a variable set outside of the critical section containing the ->count manipulation, this leads to race condition where RWSEM_FLAG_HANDOFF can be double subtracted, corrupting ->count. To make the handoff bit handling more consistent and robust, extract out handoff bit clearing code into the new rwsem_del_waiter() helper function. Also, completely eradicate wstate; always evaluate everything inside the same critical section. The common function will only use atomic_long_andnot() to clear bits when the wait queue is empty to avoid possible race condition. If the first waiter with handoff bit set is killed or interrupted to exit the slowpath without acquiring the lock, the next waiter will inherit the handoff bit. While at it, simplify the trylock for loop in rwsem_down_write_slowpath() to make it easier to read. Fixes: `4f23dbc1e6` ("locking/rwsem: Implement lock handoff to prevent lock starvation") Reported-by: Zhenhua Ma <mazhenhua@xiaomi.com> Suggested-by: Peter Zijlstra <peterz@infradead.org> Signed-off-by: Waiman Long <longman@redhat.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Link: https://lkml.kernel.org/r/20211116012912.723980-1-longman@redhat.com Signed-off-by: Sasha Levin <sashal@kernel.org>		2021-12-01 09:04:54 +01:00
arch	MIPS: use 3-level pgtable for 64KB page size on MIPS_VA_BITS_48	2021-12-01 09:04:53 +01:00
block	block: Check ADMIN before NICE for IOPRIO_CLASS_RT	2021-11-25 09:48:45 +01:00
certs	certs: Add support for using elliptic curve keys for signing modules	2021-08-23 19:55:42 +03:00
crypto	crypto: pcrypt - Delay write to padata->info	2021-11-18 19:16:44 +01:00
Documentation	netfilter: ipvs: Fix reuse connection if RS weight is 0	2021-12-01 09:04:45 +01:00
drivers	net: mscc: ocelot: correctly report the timestamping RX filters in ethtool	2021-12-01 09:04:54 +01:00
fs	erofs: fix deadlock when shrink erofs slab	2021-12-01 09:04:50 +01:00
include	net: ipv6: add fib6_nh_release_dsts stub	2021-12-01 09:04:49 +01:00
init	init: make unknown command line param message clearer	2021-11-18 19:17:11 +01:00
ipc	shm: extend forced shm destroy to support objects from several IPC nses	2021-11-25 09:48:42 +01:00
kernel	locking/rwsem: Make handoff bit handling more consistent	2021-12-01 09:04:54 +01:00
lib	printk: restore flushing of NMI buffers on remote CPUs after NMI backtraces	2021-11-25 09:48:45 +01:00
LICENSES	LICENSES/dual/CC-BY-4.0: Git rid of "smart quotes"	2021-07-15 06:31:24 -06:00
mm	hugetlbfs: flush TLBs correctly after huge_pmd_unshare	2021-11-25 09:49:07 +01:00
net	net/smc: Don't call clcsock shutdown twice when smc shutdown	2021-12-01 09:04:53 +01:00
samples	samples/bpf: Fix incorrect use of strlen in xdp_redirect_cpu	2021-11-25 09:48:33 +01:00
scripts	leaking_addresses: Always print a trailing newline	2021-11-18 19:16:16 +01:00
security	selinux: fix NULL-pointer dereference when hashtab allocation fails	2021-11-25 09:49:07 +01:00
sound	ALSA: intel-dsp-config: add quirk for JSL devices based on ES8336 codec	2021-12-01 09:04:49 +01:00
tools	tools build: Fix removal of feature-sync-compare-and-swap feature detection	2021-11-25 09:48:40 +01:00
usr	.gitignore: prefix local generated files with a slash	2021-05-02 00:43:35 +09:00
virt	KVM: Remove tlbs_dirty	2021-09-23 11:01:12 -04:00
.clang-format	clang-format: Update with the latest for_each macro list	2021-05-12 23:32:39 +02:00
.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	.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	drm fixes for 5.15 final	2021-10-28 12:17:01 -07:00
Makefile	Linux 5.15.5	2021-11-25 09:49:08 +01:00
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.