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Thomas Gleixner 20d853fd07 watchdog/hardlockup/perf: Remove broken self disable on failure
The self disabling feature is broken vs. CPU hotplug locking:

CPU 0			   CPU 1
cpus_write_lock();
 cpu_up(1)
   wait_for_completion()
			   ....
			   unpark_watchdog()
			   ->unpark()
			     perf_event_create() <- fails
			       watchdog_enable &= ~NMI_WATCHDOG;
			   ....
cpus_write_unlock();
			   CPU 2
cpus_write_lock()
 cpu_down(2)
   wait_for_completion()
			   wakeup(watchdog);
			     watchdog()
			     if (!(watchdog_enable & NMI_WATCHDOG))
				watchdog_nmi_disable()
				  perf_event_disable()
				  ....
				  cpus_read_lock();

			   stop_smpboot_threads()
			     park_watchdog();
			       wait_for_completion(watchdog->parked);

Result: End of hotplug and instantaneous full lockup of the machine.

There is a similar problem with disabling the watchdog via the user space
interface as the sysctl function fiddles with watchdog_enable directly.

It's very debatable whether this is required at all. If the watchdog works
nicely on N CPUs and it fails to enable on the N + 1 CPU either during
hotplug or because the user space interface disabled it via sysctl cpumask
and then some perf user grabbed the counter which is then unavailable for
the watchdog when the sysctl cpumask gets changed back.

There is no real justification for this.

One of the reasons WHY this is done is the utter stupidity of the init code
of the perf NMI watchdog. Instead of checking upfront at boot whether PERF
is available and functional at all, it just does this check at run time
over and over when user space fiddles with the sysctl. That's broken beyond
repair along with the idiotic error code dependent warn level printks and
the even more silly printk rate limiting.

If the init code checks whether perf works at boot time, then this mess can
be more or less avoided completely. Perf does not come magically into life
at runtime. Brain usage while coding is overrated.

Remove the cruft and add a temporary safe guard which gets removed later.

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/20170912194146.806708429@linutronix.de
Signed-off-by: Ingo Molnar <mingo@kernel.org>
2017-09-14 11:41:04 +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 broken suspend/resume interfaces 2017-09-14 11:41:04 +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/hardlockup/perf: Remove broken self disable on failure 2017-09-14 11:41:04 +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

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.