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Niklas Söderlund d4c7c28806 timekeeping: Allow runtime PM from change_clocksource() 
The struct clocksource callbacks enable() and disable() are described as a
way to allow clock sources to enter a power save mode. See commit
4614e6adaf ("clocksource: add enable() and disable() callbacks")

But using runtime PM from these callbacks triggers a cyclic lockdep warning when
switching clock source using change_clocksource().

  # echo e60f0000.timer > /sys/devices/system/clocksource/clocksource0/current_clocksource
   ======================================================
   WARNING: possible circular locking dependency detected
   ------------------------------------------------------
   migration/0/11 is trying to acquire lock:
   ffff0000403ed220 (&dev->power.lock){-...}-{2:2}, at: __pm_runtime_resume+0x40/0x74

   but task is already holding lock:
   ffff8000113c8f88 (tk_core.seq.seqcount){----}-{0:0}, at: multi_cpu_stop+0xa4/0x190

   which lock already depends on the new lock.

   the existing dependency chain (in reverse order) is:

   -> #2 (tk_core.seq.seqcount){----}-{0:0}:
          ktime_get+0x28/0xa0
          hrtimer_start_range_ns+0x210/0x2dc
          generic_sched_clock_init+0x70/0x88
          sched_clock_init+0x40/0x64
          start_kernel+0x494/0x524

   -> #1 (hrtimer_bases.lock){-.-.}-{2:2}:
          hrtimer_start_range_ns+0x68/0x2dc
          rpm_suspend+0x308/0x5dc
          rpm_idle+0xc4/0x2a4
          pm_runtime_work+0x98/0xc0
          process_one_work+0x294/0x6f0
          worker_thread+0x70/0x45c
          kthread+0x154/0x160
          ret_from_fork+0x10/0x20

   -> #0 (&dev->power.lock){-...}-{2:2}:
          _raw_spin_lock_irqsave+0x7c/0xc4
          __pm_runtime_resume+0x40/0x74
          sh_cmt_start+0x1c4/0x260
          sh_cmt_clocksource_enable+0x28/0x50
          change_clocksource+0x9c/0x160
          multi_cpu_stop+0xa4/0x190
          cpu_stopper_thread+0x90/0x154
          smpboot_thread_fn+0x244/0x270
          kthread+0x154/0x160
          ret_from_fork+0x10/0x20

   other info that might help us debug this:

   Chain exists of:
     &dev->power.lock --> hrtimer_bases.lock --> tk_core.seq.seqcount

    Possible unsafe locking scenario:

          CPU0                    CPU1
          ----                    ----
     lock(tk_core.seq.seqcount);
                                  lock(hrtimer_bases.lock);
                                  lock(tk_core.seq.seqcount);
     lock(&dev->power.lock);

    *** DEADLOCK ***

   2 locks held by migration/0/11:
    #0: ffff8000113c9278 (timekeeper_lock){-.-.}-{2:2}, at: change_clocksource+0x2c/0x160
    #1: ffff8000113c8f88 (tk_core.seq.seqcount){----}-{0:0}, at: multi_cpu_stop+0xa4/0x190

Rework change_clocksource() so it enables the new clocksource and disables
the old clocksource outside of the timekeeper_lock and seqcount write held
region. There is no requirement that these callbacks are invoked from the
lock held region.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Link: https://lore.kernel.org/r/20210211134318.323910-1-niklas.soderlund+renesas@ragnatech.se
 		2021-03-29 16:41:59 +02:00
Documentation A set of irqchip updates: 2021-03-14 13:33:33 -07:00
LICENSES LICENSES: Add the CC-BY-4.0 license 2020-12-08 10:33:27 -07:00
arch A set of irqchip updates: 2021-03-14 13:33:33 -07:00
block block: Discard page cache of zone reset target range 2021-03-11 11:49:25 -07:00
certs certs: Replace K{U,G}IDT_INIT() with GLOBAL_ROOT_{U,G}ID 2021-01-21 16:16:10 +00:00
crypto crypto: mips/poly1305 - enable for all MIPS processors 2021-03-08 11:52:17 +01:00
drivers timekeeping, clocksource: Fix various typos in comments 2021-03-22 23:06:48 +01:00
fs Merge branch 'akpm' (patches from Andrew) 2021-03-14 12:23:34 -07:00
include timekeeping, clocksource: Fix various typos in comments 2021-03-22 23:06:48 +01:00
init Merge branch 'akpm' (patches from Andrew) 2021-03-14 12:23:34 -07:00
ipc fs: make helpers idmap mount aware 2021-01-24 14:27:20 +01:00
kernel timekeeping: Allow runtime PM from change_clocksource() 2021-03-29 16:41:59 +02:00
lib kasan: fix KASAN_STACK dependency for HW_TAGS 2021-03-13 11:27:31 -08:00
mm Merge branch 'akpm' (patches from Andrew) 2021-03-14 12:23:34 -07:00
net NFS Client Bugfixes for Linux 5.10-rc3 2021-03-12 14:19:35 -08:00
samples Merge git://git.kernel.org:/pub/scm/linux/kernel/git/netdev/net 2021-03-09 17:15:56 -08:00
scripts kbuild: fix ld-version.sh to not be affected by locale 2021-03-13 11:12:13 +09:00
security Revert 95ebabde38 ("capabilities: Don't allow writing ambiguous v3 file capabilities") 2021-03-12 15:27:14 -06:00
sound ALSA: hda/hdmi: Cancel pending works before suspend 2021-03-10 12:52:01 +01:00
tools timekeeping, clocksource: Fix various typos in comments 2021-03-22 23:06:48 +01:00
usr Kbuild updates for v5.12 2021-02-25 10:17:31 -08:00
virt KVM: x86/mmu: Consider the hva in mmu_notifier retry 2021-02-22 13:16:53 -05:00
.clang-format cxl for 5.12 2021-02-24 09:38:36 -08:00
.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 clang-lto series for v5.12-rc1 2021-02-23 09:28:51 -08:00
.mailmap treewide: Miguel has moved 2021-02-26 09:41:03 -08:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS treewide: Miguel has moved 2021-02-26 09:41:03 -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 Merge branch 'akpm' (patches from Andrew) 2021-03-14 12:23:34 -07:00
Makefile Linux 5.12-rc3 2021-03-14 14:41:02 -07:00
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.