There are three subsystem callbacks in css shutdown path -
css_offline(), css_released() and css_free().  Except for
css_released(), cgroup core didn't guarantee the order of invocation.
css_offline() or css_free() could be called on a parent css before its
children.  This behavior is unexpected and led to bugs in cpu and
memory controller.

This patch updates offline path so that a parent css is never offlined
before its children.  Each css keeps online_cnt which reaches zero iff
itself and all its children are offline and offline_css() is invoked
only after online_cnt reaches zero.

This fixes the memory controller bug and allows the fix for cpu
controller.
Signed-off-by: NTejun Heo <tj@kernel.org>
Reported-and-tested-by: NChristian Borntraeger <borntraeger@de.ibm.com>
Reported-by: NBrian Christiansen <brian.o.christiansen@gmail.com>
Link: http://lkml.kernel.org/g/5698A023.9070703@de.ibm.com
Link: http://lkml.kernel.org/g/CAKB58ikDkzc8REt31WBkD99+hxNzjK4+FBmhkgS+NVrC9vjMSg@mail.gmail.com
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: stable@vger.kernel.org

sock_cgroup_data is a struct containing an anonymous union.
sock_cgroup_set_prioidx() and sock_cgroup_set_classid() were
initializing a field inside the anonymous union as follows.

 struct sock_ccgroup_data skcd_buf = { .val = VAL };

While this is fine on more recent compilers, gcc-4.4.7 triggers the
following errors.

 include/linux/cgroup-defs.h: In function ‘sock_cgroup_set_prioidx’:
 include/linux/cgroup-defs.h:619: error: unknown field ‘val’ specified in initializer
 include/linux/cgroup-defs.h:619: warning: missing braces around initializer
 include/linux/cgroup-defs.h:619: warning: (near initialization for ‘skcd_buf.<anonymous>’)

This is because .val belongs to the anonymous union nested inside the
struct but the initializer is missing the nesting.  Fix it by adding
an extra pair of braces.
Signed-off-by: NTejun Heo <tj@kernel.org>
Reported-by: NAlaa Hleihel <alaa@dev.mellanox.co.il>
Fixes: bd1060a1 ("sock, cgroup: add sock->sk_cgroup")
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

In cgroup v1, dealing with cgroup membership was difficult because the
number of membership associations was unbound.  As a result, cgroup v1
grew several controllers whose primary purpose is either tagging
membership or pull in configuration knobs from other subsystems so
that cgroup membership test can be avoided.

net_cls and net_prio controllers are examples of the latter.  They
allow configuring network-specific attributes from cgroup side so that
network subsystem can avoid testing cgroup membership; unfortunately,
these are not only cumbersome but also problematic.

Both net_cls and net_prio aren't properly hierarchical.  Both inherit
configuration from the parent on creation but there's no interaction
afterwards.  An ancestor doesn't restrict the behavior in its subtree
in anyway and configuration changes aren't propagated downwards.
Especially when combined with cgroup delegation, this is problematic
because delegatees can mess up whatever network configuration
implemented at the system level.  net_prio would allow the delegatees
to set whatever priority value regardless of CAP_NET_ADMIN and net_cls
the same for classid.

While it is possible to solve these issues from controller side by
implementing hierarchical allowable ranges in both controllers, it
would involve quite a bit of complexity in the controllers and further
obfuscate network configuration as it becomes even more difficult to
tell what's actually being configured looking from the network side.
While not much can be done for v1 at this point, as membership
handling is sane on cgroup v2, it'd be better to make cgroup matching
behave like other network matches and classifiers than introducing
further complications.

In preparation, this patch updates sock->sk_cgrp_data handling so that
it points to the v2 cgroup that sock was created in until either
net_prio or net_cls is used.  Once either of the two is used,
sock->sk_cgrp_data reverts to its previous role of carrying prioidx
and classid.  This is to avoid adding yet another cgroup related field
to struct sock.

As the mode switching can happen at most once per boot, the switching
mechanism is aimed at lowering hot path overhead.  It may leak a
finite, likely small, number of cgroup refs and report spurious
prioidx or classid on switching; however, dynamic updates of prioidx
and classid have always been racy and lossy - socks between creation
and fd installation are never updated, config changes don't update
existing sockets at all, and prioidx may index with dead and recycled
cgroup IDs.  Non-critical inaccuracies from small race windows won't
make any noticeable difference.

This patch doesn't make use of the pointer yet.  The following patch
will implement netfilter match for cgroup2 membership.

v2: Use sock_cgroup_data to avoid inflating struct sock w/ another
    cgroup specific field.

v3: Add comments explaining why sock_data_prioidx() and
    sock_data_classid() use different fallback values.
Signed-off-by: NTejun Heo <tj@kernel.org>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Daniel Wagner <daniel.wagner@bmw-carit.de>
CC: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Introduce sock->sk_cgrp_data which is a struct sock_cgroup_data.
->sk_cgroup_prioidx and ->sk_classid are moved into it.  The struct
and its accessors are defined in cgroup-defs.h.  This is to prepare
for overloading the fields with a cgroup pointer.

This patch mostly performs equivalent conversions but the followings
are noteworthy.

* Equality test before updating classid is removed from
  sock_update_classid().  This shouldn't make any noticeable
  difference and a similar test will be implemented on the helper side
  later.

* sock_update_netprioidx() now takes struct sock_cgroup_data and can
  be moved to netprio_cgroup.h without causing include dependency
  loop.  Moved.

* The dummy version of sock_update_netprioidx() converted to a static
  inline function while at it.
Signed-off-by: NTejun Heo <tj@kernel.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Now that nobody use the "priv" arg passed to can_fork/cancel_fork/fork we can
kill CGROUP_CANFORK_COUNT/SUBSYS_TAG/etc and cgrp_ss_priv[] in copy_process().
Signed-off-by: NOleg Nesterov <oleg@redhat.com>
Signed-off-by: NTejun Heo <tj@kernel.org>

Consider the following v2 hierarchy.

  P0 (+memory) --- P1 (-memory) --- A
                                 \- B
       
P0 has memory enabled in its subtree_control while P1 doesn't.  If
both A and B contain processes, they would belong to the memory css of
P1.  Now if memory is enabled on P1's subtree_control, memory csses
should be created on both A and B and A's processes should be moved to
the former and B's processes the latter.  IOW, enabling controllers
can cause atomic migrations into different csses.

The core cgroup migration logic has been updated accordingly but the
controller migration methods haven't and still assume that all tasks
migrate to a single target css; furthermore, the methods were fed the
css in which subtree_control was updated which is the parent of the
target csses.  pids controller depends on the migration methods to
move charges and this made the controller attribute charges to the
wrong csses often triggering the following warning by driving a
counter negative.

 WARNING: CPU: 1 PID: 1 at kernel/cgroup_pids.c:97 pids_cancel.constprop.6+0x31/0x40()
 Modules linked in:
 CPU: 1 PID: 1 Comm: systemd Not tainted 4.4.0-rc1+ #29
 ...
  ffffffff81f65382 ffff88007c043b90 ffffffff81551ffc 0000000000000000
  ffff88007c043bc8 ffffffff810de202 ffff88007a752000 ffff88007a29ab00
  ffff88007c043c80 ffff88007a1d8400 0000000000000001 ffff88007c043bd8
 Call Trace:
  [<ffffffff81551ffc>] dump_stack+0x4e/0x82
  [<ffffffff810de202>] warn_slowpath_common+0x82/0xc0
  [<ffffffff810de2fa>] warn_slowpath_null+0x1a/0x20
  [<ffffffff8118e031>] pids_cancel.constprop.6+0x31/0x40
  [<ffffffff8118e0fd>] pids_can_attach+0x6d/0xf0
  [<ffffffff81188a4c>] cgroup_taskset_migrate+0x6c/0x330
  [<ffffffff81188e05>] cgroup_migrate+0xf5/0x190
  [<ffffffff81189016>] cgroup_attach_task+0x176/0x200
  [<ffffffff8118949d>] __cgroup_procs_write+0x2ad/0x460
  [<ffffffff81189684>] cgroup_procs_write+0x14/0x20
  [<ffffffff811854e5>] cgroup_file_write+0x35/0x1c0
  [<ffffffff812e26f1>] kernfs_fop_write+0x141/0x190
  [<ffffffff81265f88>] __vfs_write+0x28/0xe0
  [<ffffffff812666fc>] vfs_write+0xac/0x1a0
  [<ffffffff81267019>] SyS_write+0x49/0xb0
  [<ffffffff81bcef32>] entry_SYSCALL_64_fastpath+0x12/0x76

This patch fixes the bug by removing @css parameter from the three
migration methods, ->can_attach, ->cancel_attach() and ->attach() and
updating cgroup_taskset iteration helpers also return the destination
css in addition to the task being migrated.  All controllers are
updated accordingly.

* Controllers which don't care whether there are one or multiple
  target csses can be converted trivially.  cpu, io, freezer, perf,
  netclassid and netprio fall in this category.

* cpuset's current implementation assumes that there's single source
  and destination and thus doesn't support v2 hierarchy already.  The
  only change made by this patchset is how that single destination css
  is obtained.

* memory migration path already doesn't do anything on v2.  How the
  single destination css is obtained is updated and the prep stage of
  mem_cgroup_can_attach() is reordered to accomodate the change.

* pids is the only controller which was affected by this bug.  It now
  correctly handles multi-destination migrations and no longer causes
  counter underflow from incorrect accounting.
Signed-off-by: NTejun Heo <tj@kernel.org>
Reported-and-tested-by: NDaniel Wagner <daniel.wagner@bmw-carit.de>
Cc: Aleksa Sarai <cyphar@cyphar.com>

cgroup_is_descendant() currently walks up the hierarchy and compares
each ancestor to the cgroup in question.  While enough for cgroup core
usages, this can't be used in hot paths to test cgroup membership.
This patch adds cgroup->ancestor_ids[] which records the IDs of all
ancestors including self and cgroup->level for the nesting level.

This allows testing whether a given cgroup is a descendant of another
in three finite steps - testing whether the two belong to the same
hierarchy, whether the descendant candidate is at the same or a higher
level than the ancestor and comparing the recorded ancestor_id at the
matching level.  cgroup_is_descendant() is accordingly reimplmented
and made inline.
Signed-off-by: NTejun Heo <tj@kernel.org>

With major controllers - cpu, memory and io - shaping up for the
unified hierarchy, cgroup2 is about ready to be, gradually, released
into the wild.  Replace __DEVEL__sane_behavior flag which was used to
select the unified hierarchy with a separate filesystem type "cgroup2"
so that unified hierarchy can be mounted as follows.

  mount -t cgroup2 none $MOUNT_POINT

The cgroup2 fs has its own magic number - 0x63677270 ("cgrp").

v2: Assign a different magic number to cgroup2 fs.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NLi Zefan <lizefan@huawei.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>

6f60eade ("cgroup: generalize obtaining the handles of and
notifying cgroup files") introduced cftype->file_offset so that the
handles for per-css file instances can be recorded.  These handles
then can be used, for example, to generate file modified
notifications.

Unfortunately, it made the wrong assumption that files are created
once for a given css and removed on its destruction.  Due to the
dependencies among subsystems, a css may be hidden from userland and
then later shown again.  This is implemented by removing and
re-creating the affected files, so the associated kernfs_node for a
given cgroup file may change over time.  This incorrect assumption led
to the corruption of css->files lists.

Reimplement cftype->file_offset handling so that cgroup_file->kn is
protected by a lock and updated as files are created and destroyed.
This also makes keeping them on per-cgroup list unnecessary.
Signed-off-by: NTejun Heo <tj@kernel.org>
Reported-by: NJames Sedgwick <jsedgwick@fb.com>
Fixes: 6f60eade ("cgroup: generalize obtaining the handles of and notifying cgroup files")
Acked-by: NJohannes Weiner <hannes@cmpxchg.org>
Acked-by: NZefan Li <lizefan@huawei.com>

pids controller is completely broken in that it uncharges when a task
exits allowing zombies to escape resource control.  With the recent
updates, cgroup core now maintains cgroup association till task free
and pids controller can be fixed by uncharging on free instead of
exit.

This patch adds cgroup_subsys->free() method and update pids
controller to use it instead of ->exit() for uncharging.
Signed-off-by: NTejun Heo <tj@kernel.org>
Cc: Aleksa Sarai <cyphar@cyphar.com>

cgroup_exit() is called when a task exits and disassociates the
exiting task from its cgroups and half-attach it to the root cgroup.
This is unnecessary and undesirable.

No controller actually needs an exiting task to be disassociated with
non-root cgroups.  Both cpu and perf_event controllers update the
association to the root cgroup from their exit callbacks just to keep
consistent with the cgroup core behavior.

Also, this disassociation makes it difficult to track resources held
by zombies or determine where the zombies came from.  Currently, pids
controller is completely broken as it uncharges on exit and zombies
always escape the resource restriction.  With cgroup association being
reset on exit, fixing it is pretty painful.

There's no reason to reset cgroup membership on exit.  The zombie can
be removed from its css_set so that it doesn't show up on
"cgroup.procs" and thus can't be migrated or interfere with cgroup
removal.  It can still pin and point to the css_set so that its cgroup
membership is maintained.  This patch makes cgroup core keep zombies
associated with their cgroups at the time of exit.

* Previous patches decoupled populated_cnt tracking from css_set
  lifetime, so a dying task can be simply unlinked from its css_set
  while pinning and pointing to the css_set.  This keeps css_set
  association from task side alive while hiding it from "cgroup.procs"
  and populated_cnt tracking.  The css_set reference is dropped when
  the task_struct is freed.

* ->exit() callback no longer needs the css arguments as the
  associated css never changes once PF_EXITING is set.  Removed.

* cpu and perf_events controllers no longer need ->exit() callbacks.
  There's no reason to explicitly switch away on exit.  The final
  schedule out is enough.  The callbacks are removed.

* On traditional hierarchies, nothing changes.  "/proc/PID/cgroup"
  still reports "/" for all zombies.  On the default hierarchy,
  "/proc/PID/cgroup" keeps reporting the cgroup that the task belonged
  to at the time of exit.  If the cgroup gets removed before the task
  is reaped, " (deleted)" is appended.

v2: Build brekage due to missing dummy cgroup_free() when
    !CONFIG_CGROUP fixed.
Signed-off-by: NTejun Heo <tj@kernel.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>

css_sets are synchronized through css_set_rwsem but the locking scheme
is kinda bizarre.  The hot paths - fork and exit - have to write lock
the rwsem making the rw part pointless; furthermore, many readers
already hold cgroup_mutex.

One of the readers is css task iteration.  It read locks the rwsem
over the entire duration of iteration.  This leads to silly locking
behavior.  When cpuset tries to migrate processes of a cgroup to a
different NUMA node, css_set_rwsem is held across the entire migration
attempt which can take a long time locking out forking, exiting and
other cgroup operations.

This patch updates css task iteration so that it locks css_set_rwsem
only while the iterator is being advanced.  css task iteration
involves two levels - css_set and task iteration.  As css_sets in use
are practically immutable, simply pinning the current one is enough
for resuming iteration afterwards.  Task iteration is tricky as tasks
may leave their css_set while iteration is in progress.  This is
solved by keeping track of active iterators and advancing them if
their next task leaves its css_set.

v2: put_task_struct() in css_task_iter_next() moved outside
    css_set_rwsem.  A later patch will add cgroup operations to
    task_struct free path which may grab the same lock and this avoids
    deadlock possibilities.

    css_set_move_task() updated to use list_for_each_entry_safe() when
    walking task_iters and advancing them.  This is necessary as
    advancing an iter may remove it from the list.
Signed-off-by: NTejun Heo <tj@kernel.org>

Currently, cgroup->nr_populated counts whether the cgroup has any
css_sets linked to it and the number of children which has non-zero
->nr_populated.  This works because a css_set's refcnt converges with
the number of tasks linked to it and thus there's no css_set linked to
a cgroup if it doesn't have any live tasks.

To help tracking resource usage of zombie tasks, putting the ref of
css_set will be separated from disassociating the task from the
css_set which means that a cgroup may have css_sets linked to it even
when it doesn't have any live tasks.

This patch updates cgroup->nr_populated so that for the cgroup itself
it counts the number of css_sets which have tasks associated with them
so that empty css_sets don't skew the populated test.
Signed-off-by: NTejun Heo <tj@kernel.org>

cgroup core handles creations and removals of cgroup interface files
as described by cftypes.  There are cases where the handle for a given
file instance is necessary, for example, to generate a file modified
event.  Currently, this is handled by explicitly matching the callback
method pointer and storing the file handle manually in
cgroup_add_file().  While this simple approach works for cgroup core
files, it can't for controller interface files.

This patch generalizes cgroup interface file handle handling.  struct
cgroup_file is defined and each cftype can optionally tell cgroup core
to store the file handle by setting ->file_offset.  A file handle
remains accessible as long as the containing css is accessible.

Both "cgroup.procs" and "cgroup.events" are converted to use the new
generic mechanism instead of hooking directly into cgroup_add_file().
Also, cgroup_file_notify() which takes a struct cgroup_file and
generates a file modified event on it is added and replaces explicit
kernfs_notify() invocations.

This generalizes cgroup file handle handling and allows controllers to
generate file modified notifications.
Signed-off-by: NTejun Heo <tj@kernel.org>
Cc: Li Zefan <lizefan@huawei.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>

cftype->mode allows controllers to give arbitrary permissions to
interface knobs.  Except for "cgroup.event_control", the existing uses
are spurious.

* Some explicitly specify S_IRUGO | S_IWUSR even though that's the
  default.

* "cpuset.memory_pressure" specifies S_IRUGO while also setting a
  write callback which returns -EACCES.  All it needs to do is simply
  not setting a write callback.

"cgroup.event_control" uses cftype->mode to make the file
world-writable.  It's a misdesigned interface and we don't want
controllers to be tweaking interface file permissions in general.
This patch removes cftype->mode and all its spurious uses and
implements CFTYPE_WORLD_WRITABLE for "cgroup.event_control" which is
marked as compatibility-only.
Signed-off-by: NTejun Heo <tj@kernel.org>
Cc: Li Zefan <lizefan@huawei.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>

memcg already uses "memory.events" for event reporting and other
controllers may need event reporting too.  Let's standardize on
"$SUBSYS.events" interface file for reporting events which don't
happen too frequently and thus can share event notification.

"cgroup.populated" is replaced with "populated" field in
"cgroup.events" and documentation is updated accordingly.
Signed-off-by: NTejun Heo <tj@kernel.org>
Cc: Li Zefan <lizefan@huawei.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>

Replace cgroup_subsys->disabled tests in controllers with
cgroup_subsys_enabled().  cgroup_subsys_enabled() requires literal
subsys name as its parameter and thus can't be used for cgroup core
which iterates through controllers.  For cgroup core, introduce and
use cgroup_ssid_enabled() which uses slower static_key_enabled() test
and can be indexed by subsys ID.

This leaves cgroup_subsys->disabled unused.  Removed.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NZefan Li <lizefan@huawei.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>

Note: This commit was originally committed as d59cfc09 but got
      reverted by 0c986253 due to the performance regression from
      the percpu_rwsem write down/up operations added to cgroup task
      migration path.  percpu_rwsem changes which alleviate the
      performance issue are pending for v4.4-rc1 merge window.
      Re-apply.

The cgroup side of threadgroup locking uses signal_struct->group_rwsem
to synchronize against threadgroup changes.  This per-process rwsem
adds small overhead to thread creation, exit and exec paths, forces
cgroup code paths to do lock-verify-unlock-retry dance in a couple
places and makes it impossible to atomically perform operations across
multiple processes.

This patch replaces signal_struct->group_rwsem with a global
percpu_rwsem cgroup_threadgroup_rwsem which is cheaper on the reader
side and contained in cgroups proper.  This patch converts one-to-one.

This does make writer side heavier and lower the granularity; however,
cgroup process migration is a fairly cold path, we do want to optimize
thread operations over it and cgroup migration operations don't take
enough time for the lower granularity to matter.
Signed-off-by: NTejun Heo <tj@kernel.org>
Link: http://lkml.kernel.org/g/55F8097A.7000206@de.ibm.com
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>

This reverts commit d59cfc09.

d59cfc09 ("sched, cgroup: replace signal_struct->group_rwsem with
a global percpu_rwsem") and b5ba75b5 ("cgroup: simplify
threadgroup locking") changed how cgroup synchronizes against task
fork and exits so that it uses global percpu_rwsem instead of
per-process rwsem; unfortunately, the write [un]lock paths of
percpu_rwsem always involve synchronize_rcu_expedited() which turned
out to be too expensive.

Improvements for percpu_rwsem are scheduled to be merged in the coming
v4.4-rc1 merge window which alleviates this issue.  For now, revert
the two commits to restore per-process rwsem.  They will be re-applied
for the v4.4-rc1 merge window.
Signed-off-by: NTejun Heo <tj@kernel.org>
Link: http://lkml.kernel.org/g/55F8097A.7000206@de.ibm.comReported-by: NChristian Borntraeger <borntraeger@de.ibm.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: stable@vger.kernel.org # v4.2+

This allows cgroup subsystems to use a different name on the unified
hierarchy.  cgroup_subsys->name is used on the unified hierarchy,
->legacy_name elsewhere.  If ->legacy_name is not explicitly set, it's
automatically set to ->name and the userland visible behavior remains
unchanged.

v2: Make parse_cgroupfs_options() only consider ->legacy_name as mount
    options are used only on legacy hierarchies.  Suggested by Li
    Zefan.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NLi Zefan <lizefan@huawei.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: cgroups@vger.kernel.org

It's pretty unusual to have an int as a private data field and it
makes it impossible to carray a pointer value through it.  Let's make
it an unsigned long.  AFAICS, this shouldn't break anything.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NLi Zefan <lizefan@huawei.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>

Add a new cgroup subsystem callback can_fork that conditionally
states whether or not the fork is accepted or rejected by a cgroup
policy. In addition, add a cancel_fork callback so that if an error
occurs later in the forking process, any state modified by can_fork can
be reverted.

Allow for a private opaque pointer to be passed from cgroup_can_fork to
cgroup_post_fork, allowing for the fork state to be stored by each
subsystem separately.

Also add a tagging system for cgroup_subsys.h to allow for CGROUP_<TAG>
enumerations to be be defined and used. In addition, explicitly add a
CGROUP_CANFORK_COUNT macro to make arrays easier to define.

This is in preparation for implementing the pids cgroup subsystem.
Signed-off-by: NAleksa Sarai <cyphar@cyphar.com>
Signed-off-by: NTejun Heo <tj@kernel.org>

On traditional hierarchies, if a task has write access to "tasks" or
"cgroup.procs" file of a cgroup and its euid agrees with the target,
it can move the target to the cgroup; however, consider the following
scenario.  The owner of each cgroup is in the parentheses.

 R (root) - 0 (root) - 00 (user1) - 000 (user1)
          |                       \ 001 (user1)
          \ 1 (root) - 10 (user1)

The subtrees of 00 and 10 are delegated to user1; however, while both
subtrees may belong to the same user, it is clear that the two
subtrees are to be isolated - they're under completely separate
resource limits imposed by 0 and 1, respectively.  Note that 0 and 1
aren't strictly necessary but added to ease illustrating the issue.

If user1 is allowed to move processes between the two subtrees, the
intention of the hierarchy - keeping a given group of processes under
a subtree with certain resource restrictions while delegating
management of the subtree - can be circumvented by user1.

This happens because migration permission check doesn't consider the
hierarchical nature of cgroups.  To fix the issue, this patch adds an
extra permission requirement when userland tries to migrate a process
in the default hierarchy - the issuing task must have write access to
the common ancestor of "cgroup.procs" file of the ancestor in addition
to the destination's.

Conceptually, the issuer must be able to move the target process from
the source cgroup to the common ancestor of source and destination
cgroups and then to the destination.  As long as delegation is done in
a proper top-down way, this guarantees that a delegatee can't smuggle
processes across disjoint delegation domains.

The next patch will add documentation on the delegation model on the
default hierarchy.

v2: Fixed missing !ret test.  Spotted by Li Zefan.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NJohannes Weiner <hannes@cmpxchg.org>
Cc: Li Zefan <lizefan@huawei.com>

Add a new macro for_each_subsys_which that allows all enabled cgroup
subsystems to be filtered by a bitmask, such that mask & (1 << ssid)
determines if the subsystem is to be processed in the loop body (where
ssid is the unique id of the subsystem).

Also replace the need_forkexit_callback with two separate bitmasks for
each callback to make (ss->{fork,exit}) checks unnecessary.

tj: add a short comment for "if (!CGROUP_SUBSYS_COUNT)".
Signed-off-by: NAleksa Sarai <cyphar@cyphar.com>

Recent header file changes for cgroup caused lots of warnings
about a missing struct seq_file form declaration for every
inclusion of include/linux/cgroup-defs.h.

As some files are built with -Werror, this leads to build
failure like:

                 from /git/arm-soc/drivers/gpu/drm/tilcdc/tilcdc_crtc.c:18:
/git/arm-soc/include/linux/cgroup-defs.h:354:25: error: 'struct seq_file' declared inside parameter list [-Werror]
cc1: all warnings being treated as errors
make[6]: *** [drivers/gpu/drm/tilcdc/tilcdc_crtc.o] Error 1

This patch adds the declaration, which resolves both the
warnings and the drm failure.

tj: Moved it where other type declarations are.
Signed-off-by: NArnd Bergmann <arnd@arndb.de>
Fixes: b4a04ab7 ("cgroup: separate out include/linux/cgroup-defs.h")
Signed-off-by: NTejun Heo <tj@kernel.org>

The cgroup side of threadgroup locking uses signal_struct->group_rwsem
to synchronize against threadgroup changes.  This per-process rwsem
adds small overhead to thread creation, exit and exec paths, forces
cgroup code paths to do lock-verify-unlock-retry dance in a couple
places and makes it impossible to atomically perform operations across
multiple processes.

This patch replaces signal_struct->group_rwsem with a global
percpu_rwsem cgroup_threadgroup_rwsem which is cheaper on the reader
side and contained in cgroups proper.  This patch converts one-to-one.

This does make writer side heavier and lower the granularity; however,
cgroup process migration is a fairly cold path, we do want to optimize
thread operations over it and cgroup migration operations don't take
enough time for the lower granularity to matter.
Signed-off-by: NTejun Heo <tj@kernel.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>

threadgroup_change_begin/end() are used to mark the beginning and end
of threadgroup modifying operations to allow code paths which require
a threadgroup to stay stable across blocking operations to synchronize
against those sections using threadgroup_lock/unlock().

It's currently implemented as a general mechanism in sched.h using
per-signal_struct rwsem; however, this never grew non-cgroup use cases
and becomes noop if !CONFIG_CGROUPS.  It turns out that cgroups is
gonna be better served with a different sycnrhonization scheme and is
a bit silly to keep cgroups specific details as a general mechanism.

What's general here is identifying the places where threadgroups are
modified.  This patch restructures threadgroup locking so that
threadgroup_change_begin/end() become a place where subsystems which
need to sycnhronize against threadgroup changes can hook into.

cgroup_threadgroup_change_begin/end() which operate on the
per-signal_struct rwsem are created and threadgroup_lock/unlock() are
moved to cgroup.c and made static.

This is pure reorganization which doesn't cause any functional
changes.
Signed-off-by: NTejun Heo <tj@kernel.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>

From 2d728f74bfc071df06773e2fd7577dd5dab6425d Mon Sep 17 00:00:00 2001
From: Tejun Heo <tj@kernel.org>
Date: Wed, 13 May 2015 15:37:01 -0400

This patch separates out cgroup-defs.h from cgroup.h which has grown a
lot of dependencies.  cgroup-defs.h currently only contains constant
and type definitions and can be used to break circular include
dependency.  While moving, definitions are reordered so that
cgroup-defs.h has consistent logical structure.

This patch is pure reorganization.
Signed-off-by: NTejun Heo <tj@kernel.org>