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Tejun Heo 610467270f cgroup: don't call migration methods if there are no tasks to migrate 
Subsystem migration methods shouldn't be called for empty migrations.
cgroup_migrate_execute() implements this guarantee by bailing early if
there are no source css_sets.  This used to be correct before
a79a908fd2 ("cgroup: introduce cgroup namespaces"), but no longer
since the commit because css_sets can stay pinned without tasks in
them.

This caused cgroup_migrate_execute() call into cpuset migration
methods with an empty cgroup_taskset.  cpuset migration methods
correctly assume that cgroup_taskset_first() never returns NULL;
however, due to the bug, it can, leading to the following oops.

  Unable to handle kernel paging request for data at address 0x00000960
  Faulting instruction address: 0xc0000000001d6868
  Oops: Kernel access of bad area, sig: 11 [#1]
  ...
  CPU: 14 PID: 16947 Comm: kworker/14:0 Tainted: G        W
  4.12.0-rc4-next-20170609 #2
  Workqueue: events cpuset_hotplug_workfn
  task: c00000000ca60580 task.stack: c00000000c728000
  NIP: c0000000001d6868 LR: c0000000001d6858 CTR: c0000000001d6810
  REGS: c00000000c72b720 TRAP: 0300   Tainted: GW (4.12.0-rc4-next-20170609)
  MSR: 8000000000009033 <SF,EE,ME,IR,DR,RI,LE>  CR: 44722422  XER: 20000000
  CFAR: c000000000008710 DAR: 0000000000000960 DSISR: 40000000 SOFTE: 1
  GPR00: c0000000001d6858 c00000000c72b9a0 c000000001536e00 0000000000000000
  GPR04: c00000000c72b9c0 0000000000000000 c00000000c72bad0 c000000766367678
  GPR08: c000000766366d10 c00000000c72b958 c000000001736e00 0000000000000000
  GPR12: c0000000001d6810 c00000000e749300 c000000000123ef8 c000000775af4180
  GPR16: 0000000000000000 0000000000000000 c00000075480e9c0 c00000075480e9e0
  GPR20: c00000075480e8c0 0000000000000001 0000000000000000 c00000000c72ba20
  GPR24: c00000000c72baa0 c00000000c72bac0 c000000001407248 c00000000c72ba20
  GPR28: c00000000141fc80 c00000000c72bac0 c00000000c6bc790 0000000000000000
  NIP [c0000000001d6868] cpuset_can_attach+0x58/0x1b0
  LR [c0000000001d6858] cpuset_can_attach+0x48/0x1b0
  Call Trace:
  [c00000000c72b9a0] [c0000000001d6858] cpuset_can_attach+0x48/0x1b0 (unreliable)
  [c00000000c72ba00] [c0000000001cbe80] cgroup_migrate_execute+0xb0/0x450
  [c00000000c72ba80] [c0000000001d3754] cgroup_transfer_tasks+0x1c4/0x360
  [c00000000c72bba0] [c0000000001d923c] cpuset_hotplug_workfn+0x86c/0xa20
  [c00000000c72bca0] [c00000000011aa44] process_one_work+0x1e4/0x580
  [c00000000c72bd30] [c00000000011ae78] worker_thread+0x98/0x5c0
  [c00000000c72bdc0] [c000000000124058] kthread+0x168/0x1b0
  [c00000000c72be30] [c00000000000b2e8] ret_from_kernel_thread+0x5c/0x74
  Instruction dump:
  f821ffa1 7c7d1b78 60000000 60000000 38810020 7fa3eb78 3f42ffed 4bff4c25
  60000000 3b5a0448 3d420020 eb610020 <e9230960> 7f43d378 e9290000 f92af200
  ---[ end trace dcaaf98fb36d9e64 ]---

This patch fixes the bug by adding an explicit nr_tasks counter to
cgroup_taskset and skipping calling the migration methods if the
counter is zero.  While at it, remove the now spurious check on no
source css_sets.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-and-tested-by: Abdul Haleem <abdhalee@linux.vnet.ibm.com>
Cc: Roman Gushchin <guro@fb.com>
Cc: stable@vger.kernel.org # v4.6+
Fixes: a79a908fd2 ("cgroup: introduce cgroup namespaces")
Link: http://lkml.kernel.org/r/1497266622.15415.39.camel@abdul.in.ibm.com
 		2017-07-08 07:37:50 -04:00
Documentation Writeback error handling fixes (pile #2) 2017-07-07 19:38:17 -07:00
arch Kbuild thin archives updates for v4.13 2017-07-07 15:11:12 -07:00
block Merge branch 'misc.compat' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs 2017-07-06 20:57:13 -07:00
certs scripts/spelling.txt: add "intialise(d)" pattern and fix typo instances 2017-05-08 17:15:13 -07:00
crypto Merge git://git.kernel.org/pub/scm/linux/kernel/git/davem/net-next 2017-07-05 12:31:59 -07:00
drivers Writeback error handling fixes (pile #2) 2017-07-07 19:38:17 -07:00
firmware firmware/Makefile: force recompilation if makefile changes 2017-05-08 17:15:10 -07:00
fs Merge branch 'work.read_write' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs 2017-07-07 21:48:15 -07:00
include Merge branch 'uaccess-work.iov_iter' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs 2017-07-07 20:39:20 -07:00
init mm: allow slab_nomerge to be set at build time 2017-07-06 16:24:31 -07:00
ipc Writeback error handling fixes (pile #1) 2017-07-07 18:39:15 -07:00
kernel cgroup: don't call migration methods if there are no tasks to migrate 2017-07-08 07:37:50 -04:00
lib Merge branch 'uaccess-work.iov_iter' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs 2017-07-07 20:39:20 -07:00
mm Writeback error handling fixes (pile #2) 2017-07-07 19:38:17 -07:00
net Merge branch 'work.memdup_user' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs 2017-07-05 16:05:24 -07:00
samples bpf: fix return in load_bpf_file 2017-07-05 09:05:28 +01:00
scripts Kbuild thin archives updates for v4.13 2017-07-07 15:11:12 -07:00
security Merge branch 'work.memdup_user' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs 2017-07-05 16:05:24 -07:00
sound sound updates for 4.13-rc1 2017-07-06 10:56:51 -07:00
tools Kbuild updates for v4.13 2017-07-07 14:09:24 -07:00
usr ramfs: clarify help text that compression applies to ramfs as well as legacy ramdisk. 2017-07-06 16:24:30 -07:00
virt PPC: 2017-07-06 18:38:31 -07:00
.cocciconfig scripts: add Linux .cocciconfig for coccinelle 2016-07-22 12:13:39 +02:00
.get_maintainer.ignore Add hch to .get_maintainer.ignore 2015-08-21 14:30:10 -07:00
.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
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CREDITS avr32: remove support for AVR32 architecture 2017-05-01 09:27:15 +02:00
Kbuild kbuild: Consolidate header generation from ASM offset information 2017-04-13 05:43:37 +09:00
Kconfig
…
MAINTAINERS Writeback error handling fixes (pile #2) 2017-07-07 19:38:17 -07:00
Makefile Kbuild thin archives updates for v4.13 2017-07-07 15:11:12 -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.