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Pavel Tikhomirov 749860ce24 prctl: propagate has_child_subreaper flag to every descendant
If process forks some children when it has is_child_subreaper
flag enabled they will inherit has_child_subreaper flag - first
group, when is_child_subreaper is disabled forked children will
not inherit it - second group. So child-subreaper does not reparent
all his descendants when their parents die. Having these two
differently behaving groups can lead to confusion. Also it is
a problem for CRIU, as when we restore process tree we need to
somehow determine which descendants belong to which group and
much harder - to put them exactly to these group.

To simplify these we can add a propagation of has_child_subreaper
flag on PR_SET_CHILD_SUBREAPER, walking all descendants of child-
subreaper to setup has_child_subreaper flag.

In common cases when process like systemd first sets itself to
be a child-subreaper and only after that forks its services, we will
have zero-length list of descendants to walk. Testing with binary
subtree of 2^15 processes prctl took < 0.007 sec and has shown close
to linear dependency(~0.2 * n * usec) on lower numbers of processes.

Moreover, I doubt someone intentionaly pre-forks the children whitch
should reparent to init before becoming subreaper, because some our
ancestor migh have had is_child_subreaper flag while forking our
sub-tree and our childs will all inherit has_child_subreaper flag,
and we have no way to influence it. And only way to check if we have
no has_child_subreaper flag is to create some childs, kill them and
see where they will reparent to.

Using walk_process_tree helper to walk subtree, thanks to Oleg! Timing
seems to be the same.

Optimize:

a) When descendant already has has_child_subreaper flag all his subtree
has it too already.

* for a) to be true need to move has_child_subreaper inheritance under
the same tasklist_lock with adding task to its ->real_parent->children
as without it process can inherit zero has_child_subreaper, then we
set 1 to it's parent flag, check that parent has no more children, and
only after child with wrong flag is added to the tree.

* Also make these inheritance more clear by using real_parent instead of
current, as on clone(CLONE_PARENT) if current has is_child_subreaper
and real_parent has no is_child_subreaper or has_child_subreaper, child
will have has_child_subreaper flag set without actually having a
subreaper in it's ancestors.

b) When some descendant is child_reaper, it's subtree is in different
pidns from us(original child-subreaper) and processes from other pidns
will never reparent to us.

So we can skip their(a,b) subtree from walk.

v2: switch to walk_process_tree() general helper, move
has_child_subreaper inheritance
v3: remove csr_descendant leftover, change current to real_parent
in has_child_subreaper inheritance
v4: small commit message fix

Fixes: ebec18a6d3 ("prctl: add PR_{SET,GET}_CHILD_SUBREAPER to allow simple process supervision")
Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
2017-02-03 16:55:15 +13:00
Documentation Documentation/unaligned-memory-access.txt: fix incorrect comparison operator 2016-12-27 13:08:42 -07:00
arch mm: optimize PageWaiters bit use for unlock_page() 2016-12-29 11:03:15 -08:00
block ktime: Cleanup ktime_set() usage 2016-12-25 17:21:22 +01:00
certs certs: Add a secondary system keyring that can be added to dynamically 2016-04-11 22:48:09 +01:00
crypto crypto: testmgr - Use heap buffer for acomp test input 2016-12-27 17:32:11 +08:00
drivers Merge branch 'linus' of git://git.kernel.org/pub/scm/linux/kernel/git/herbert/crypto-2.6 2016-12-27 17:51:36 -08:00
firmware WHENCE: use https://linuxtv.org for LinuxTV URLs 2015-12-04 10:35:11 -02:00
fs Merge branch 'nsfs-discovery' 2017-02-03 14:53:27 +13:00
include introduce the walk_process_tree() helper 2017-02-03 16:34:41 +13:00
init mm: add PageWaiters indicating tasks are waiting for a page bit 2016-12-25 11:54:48 -08:00
ipc ipc/sem: avoid idr tree lookup for interrupted semop 2016-12-14 16:04:08 -08:00
kernel prctl: propagate has_child_subreaper flag to every descendant 2017-02-03 16:55:15 +13:00
lib Merge branch 'timers-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2016-12-25 14:30:04 -08:00
mm Merge branch 'libnvdimm-fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/nvdimm/nvdimm 2017-01-01 12:27:05 -08:00
net Merge git://git.kernel.org/pub/scm/linux/kernel/git/davem/net 2016-12-27 16:04:37 -08:00
samples Merge branch 'perf-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2016-12-23 16:49:12 -08:00
scripts Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/jmorris/linux-security 2016-12-22 10:03:52 -08:00
security exec: Remove LSM_UNSAFE_PTRACE_CAP 2017-01-24 12:03:08 +13:00
sound ktime: Cleanup ktime_set() usage 2016-12-25 17:21:22 +01:00
tools Merge branch 'turbostat' of git://git.kernel.org/pub/scm/linux/kernel/git/lenb/linux 2016-12-25 14:01:28 -08:00
usr initramfs: allow again choice of the embedded initram compression algorithm 2016-12-14 16:04:08 -08:00
virt Merge branch 'timers-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2016-12-25 14:30:04 -08:00
.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 Merge branch 'misc' of git://git.kernel.org/pub/scm/linux/kernel/git/mmarek/kbuild 2016-08-02 16:48:52 -04:00
.mailmap Merge branch 'upstream' of git://git.linux-mips.org/pub/scm/ralf/upstream-linus 2016-10-15 09:26:12 -07:00
COPYING
CREDITS CREDITS: Remove outdated address information 2016-12-21 15:21:29 -08:00
Kbuild scripts/gdb: provide linux constants 2016-05-23 17:04:14 -07:00
Kconfig
MAINTAINERS Watchdog updates for v4.10 2016-12-24 11:27:45 -08:00
Makefile Linux 4.10-rc2 2017-01-01 14:31:53 -08:00
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.