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Waiman Long 2f06f70292 locking/rwsem: Prevent potential lock starvation
The lock handoff bit is added in commit 4f23dbc1e6 ("locking/rwsem:
Implement lock handoff to prevent lock starvation") to avoid lock
starvation. However, allowing readers to do optimistic spinning does
introduce an unlikely scenario where lock starvation can happen.

The lock handoff bit may only be set when a waiter is being woken up.
In the case of reader unlock, wakeup happens only when the reader count
reaches 0. If there is a continuous stream of incoming readers acquiring
read lock via optimistic spinning, it is possible that the reader count
may never reach 0 and so the handoff bit will never be asserted.

One way to prevent this scenario from happening is to disallow optimistic
spinning if the rwsem is currently owned by readers. If the previous
or current owner is a writer, optimistic spinning will be allowed.

If the previous owner is a reader but the reader count has reached 0
before, a wakeup should have been issued. So the handoff mechanism
will be kicked in to prevent lock starvation. As a result, it should
be OK to do optimistic spinning in this case.

This patch may have some impact on reader performance as it reduces
reader optimistic spinning especially if the lock critical sections
are short the number of contending readers are small.

Signed-off-by: Waiman Long <longman@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Davidlohr Bueso <dbueso@suse.de>
Link: https://lkml.kernel.org/r/20201121041416.12285-3-longman@redhat.com
2020-12-09 17:08:48 +01:00
Documentation Merge branch 'locking/rwsem' 2020-12-09 17:08:45 +01:00
LICENSES LICENSES/deprecated: add Zlib license text 2020-09-16 14:33:49 +02:00
arch arm64 fixes for -rc7 2020-12-02 12:27:37 -08:00
block block/keyslot-manager: prevent crash when num_slots=1 2020-11-20 11:52:52 -07:00
certs .gitignore: add SPDX License Identifier 2020-03-25 11:50:48 +01:00
crypto drivers-5.10-2020-10-12 2020-10-13 13:04:41 -07:00
drivers vdpa: last minute bugfixes 2020-12-02 12:20:11 -08:00
fs Various gfs2 fixes 2020-12-02 17:25:23 -08:00
include Merge branch 'locking/rwsem' 2020-12-09 17:08:45 +01:00
init Have bootconfig size and checksum be little endian 2020-12-02 12:09:36 -08:00
ipc ipc: adjust proc_ipc_sem_dointvec definition to match prototype 2020-09-05 12:14:29 -07:00
kernel locking/rwsem: Prevent potential lock starvation 2020-12-09 17:08:48 +01:00
lib Merge branch 'locking/rwsem' 2020-12-09 17:08:45 +01:00
mm mm: fix VM_BUG_ON(PageTail) and BUG_ON(PageWriteback) 2020-11-24 15:23:19 -08:00
net linux-can-fixes-for-5.10-20201127 2020-11-27 11:13:39 -08:00
samples Tracing fixes for 5.10-rc6 2020-12-01 15:30:18 -08:00
scripts Kbuild fixes for v5.10 2020-11-28 10:42:30 -08:00
security selinux/stable-5.10 PR 20201113 2020-11-14 12:04:02 -08:00
sound ALSA: hda/realtek: Add mute LED quirk to yet another HP x360 model 2020-11-28 10:00:51 +01:00
tools Have bootconfig size and checksum be little endian 2020-12-02 12:09:36 -08:00
usr Merge branch 'work.fdpic' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs 2020-08-07 13:29:39 -07:00
virt kvm: x86/mmu: Support dirty logging for the TDP MMU 2020-10-23 03:42:13 -04:00
.clang-format RDMA 5.10 pull request 2020-10-17 11:18:18 -07:00
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COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS Samsung changes for v5.10 2020-11-13 13:16:27 +01:00
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 Merge branch 'locking/rwsem' 2020-12-09 17:08:45 +01:00
Makefile Linux 5.10-rc6 2020-11-29 15:50:50 -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.