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Thomas Gleixner 01f0a02701 watchdog/core: Remove the park_in_progress obfuscation 
Commit:

  b94f51183b ("kernel/watchdog: prevent false hardlockup on overloaded system")

tries to fix the following issue:

proc_write()
   set_sample_period()    <--- New sample period becoms visible
			  <----- Broken starts
   proc_watchdog_update()
     watchdog_enable_all_cpus()		watchdog_hrtimer_fn()
     update_watchdog_all_cpus()		   restart_timer(sample_period)
        watchdog_park_threads()

					thread->park()
					  disable_nmi()
			  <----- Broken ends

The reason why this is broken is that the update of the watchdog threshold
becomes immediately effective and visible for the hrtimer function which
uses that value to rearm the timer. But the NMI/perf side still uses the
old value up to the point where it is disabled. If the rate has been
lowered then the NMI can run fast enough to 'detect' a hard lockup because
the timer has not fired due to the longer period.

The patch 'fixed' this by adding a variable:

proc_write()
   set_sample_period()
					<----- Broken starts
   proc_watchdog_update()
     watchdog_enable_all_cpus()		watchdog_hrtimer_fn()
     update_watchdog_all_cpus()		   restart_timer(sample_period)
         watchdog_park_threads()
	  park_in_progress = 1
					<----- Broken ends
				        nmi_watchdog()
					  if (park_in_progress)
					     return;

The only effect of this variable was to make the window where the breakage
can hit small enough that it was not longer observable in testing. From a
correctness point of view it is a pointless bandaid which merily papers
over the root cause: the unsychronized update of the variable.

Looking deeper into the related code pathes unearthed similar problems in
the watchdog_start()/stop() functions.

 watchdog_start()
	perf_nmi_event_start()
	hrtimer_start()

 watchdog_stop()
	hrtimer_cancel()
	perf_nmi_event_stop()

In both cases the call order is wrong because if the tasks gets preempted
or the VM gets scheduled out long enough after the first call, then there is
a chance that the next NMI will see a stale hrtimer interrupt count and
trigger a false positive hard lockup splat.

Get rid of park_in_progress so the code can be gradually deobfuscated and
pruned from several layers of duct tape papering over the root cause,
which has been either ignored or not understood at all.

Once this is removed the underlying problem will be fixed by rewriting the
proc interface to do a proper synchronized update.

Address the start/stop() ordering problem as well by reverting the call
order, so this part is at least correct now.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Don Zickus <dzickus@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Chris Metcalf <cmetcalf@mellanox.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sebastian Siewior <bigeasy@linutronix.de>
Cc: Ulrich Obergfell <uobergfe@redhat.com>
Link: http://lkml.kernel.org/r/alpine.DEB.2.20.1709052038270.2393@nanos
Signed-off-by: Ingo Molnar <mingo@kernel.org>
2017-09-14 11:41:05 +02:00
arch watchdog/core: Remove broken suspend/resume interfaces 2017-09-14 11:41:04 +02:00
block block: directly insert blk-mq request from blk_insert_cloned_request() 2017-09-11 16:43:57 -06:00
certs modsign: add markers to endif-statements in certs/Makefile 2017-07-14 11:01:37 +10:00
crypto crypto: af_alg - get_page upon reassignment to TX SGL 2017-08-22 15:03:27 +08:00
Documentation The diff is dominated by the Allwinner A10/A20 SoCs getting converted to 2017-09-13 11:04:14 -07:00
drivers The diff is dominated by the Allwinner A10/A20 SoCs getting converted to 2017-09-13 11:04:14 -07:00
firmware firmware/Makefile: force recompilation if makefile changes 2017-05-08 17:15:10 -07:00
fs Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/mszeredi/fuse 2017-09-13 10:10:19 -07:00
include watchdog/core: Remove the park_in_progress obfuscation 2017-09-14 11:41:05 +02:00
init init/main.c: extract early boot entropy from the passed cmdline 2017-09-08 18:26:50 -07:00
ipc ipc: optimize semget/shmget/msgget for lots of keys 2017-09-08 18:26:51 -07:00
kernel watchdog/core: Remove the park_in_progress obfuscation 2017-09-14 11:41:05 +02:00
lib libnvdimm for 4.14 2017-09-11 13:10:57 -07:00
mm mm/backing-dev.c: fix an error handling path in 'cgwb_create()' 2017-09-11 14:16:44 -06:00
net The highlights include: 2017-09-12 20:03:53 -07:00
samples media updates for v4.14-rc1 2017-09-07 12:53:14 -07:00
scripts Modules updates for v4.14 2017-09-13 11:28:19 -07:00
security selinux/stable-4.14 PR 20170831 2017-09-12 13:21:00 -07:00
sound The diff is dominated by the Allwinner A10/A20 SoCs getting converted to 2017-09-13 11:04:14 -07:00
tools perf/urgent fixes: 2017-09-13 09:25:10 +02:00
usr ramfs: clarify help text that compression applies to ramfs as well as legacy ramdisk. 2017-07-06 16:24:30 -07:00
virt First batch of KVM changes for 4.14 2017-09-08 15:18:36 -07:00
.cocciconfig scripts: add Linux .cocciconfig for coccinelle 2016-07-22 12:13:39 +02:00
.get_maintainer.ignore
.gitattributes .gitattributes: set git diff driver for C source code files 2016-10-07 18:46:30 -07:00
.gitignore kbuild: Add support to generate LLVM assembly files 2017-04-25 08:13:52 +09:00
.mailmap power supply and reset changes for the v4.12 series (part 2) 2017-05-12 12:02:21 -07:00
COPYING
CREDITS selinux/stable-4.14 PR 20170831 2017-09-12 13:21:00 -07:00
Kbuild kbuild: Consolidate header generation from ASM offset information 2017-04-13 05:43:37 +09:00
Kconfig
MAINTAINERS Merge branch 'x86-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2017-09-13 11:56:16 -07:00
Makefile Merge branch 'docs-next' of git://git.lwn.net/linux 2017-09-03 21:07:29 -07:00
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.