This enables us to lookup a cgroup by its id.

v4:
- add a comment for idr_remove() in cgroup_offline_fn().

v3:
- on success, idr_alloc() returns the id but not 0, so fix the BUG_ON()
  in cgroup_init().
- pass the right value to idr_alloc() so that the id for dummy cgroup is 0.
Signed-off-by: NLi Zefan <lizefan@huawei.com>
Reviewed-by: NMichal Hocko <mhocko@suse.cz>
Signed-off-by: NTejun Heo <tj@kernel.org>

Constantly use @cset for css_set variables and use @cgrp as cgroup
variables.
Signed-off-by: NLi Zefan <lizefan@huawei.com>
Signed-off-by: NTejun Heo <tj@kernel.org>

We can use struct cfent instead.

v2:
- remove cgroup_seqfile_release().
Signed-off-by: NLi Zefan <lizefan@huawei.com>
Signed-off-by: NTejun Heo <tj@kernel.org>

This should have been removed in commit d7eeac19
("cgroup: hold cgroup_mutex before calling css_offline").

While at it, update the comments.
Signed-off-by: NLi Zefan <lizefan@huawei.com>
Signed-off-by: NTejun Heo <tj@kernel.org>

rebind_subsystems() performs santiy checks even on subsystems which
aren't specified to be added or removed and the checks aren't all that
useful given that these are in a very cold path while the violations
they check would trip up in much hotter paths.

Let's remove these from rebind_subsystems().
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NLi Zefan <lizefan@huawei.com>

Module ref handling in cgroup is rather weird.
parse_cgroupfs_options() grabs all the modules for the specified
subsystems.  A module ref is kept if the specified subsystem is newly
bound to the hierarchy.  If not, or the operation fails, the refs are
dropped.  This scatters module ref handling across multiple functions
making it difficult to track.  It also make the function nasty to use
for dynamic subsystem binding which is necessary for the planned
unified hierarchy.

There's nothing which requires the subsystem modules to be pinned
between parse_cgroupfs_options() and rebind_subsystems() in both mount
and remount paths.  parse_cgroupfs_options() can just parse and
rebind_subsystems() can handle pinning the subsystems that it wants to
bind, which is a natural part of its task - binding - anyway.

Move module ref handling into rebind_subsystems() which makes the code
a lot simpler - modules are gotten iff it's gonna be bound and put iff
unbound or binding fails.

v2: Li pointed out that if a controller module is unloaded between
    parsing and binding, rebind_subsystems() won't notice the missing
    controller as it only iterates through existing controllers.  Fix
    it by updating rebind_subsystems() to compare @added_mask to
    @pinned and fail with -ENOENT if they don't match.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NLi Zefan <lizefan@huawei.com>

rebind_subsystems() currently fails if the hierarchy has any !root
cgroups; however, on the planned unified hierarchy,
rebind_subsystems() will be used while populated.  Move the test to
cgroup_remount(), which is the only place the test is necessary
anyway.

As it's impossible for the other two callers of rebind_subsystems() to
have populated hierarchy, this doesn't make any behavior changes.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NLi Zefan <lizefan@huawei.com>

Currently, creating and removing cgroup files in the root directory
are handled separately from the actual subsystem binding and unbinding
which happens in rebind_subsystems().  Also, rebind_subsystems() users
aren't handling file creation errors properly.  Let's integrate
top_cgroup file handling into rebind_subsystems() so that it's simpler
to use and everyone handles file creation errors correctly.

* On a successful return, rebind_subsystems() is guaranteed to have
  created all files of the new subsystems and deleted the ones
  belonging to the removed subsystems.  After a failure, no file is
  created or removed.

* cgroup_remount() no longer needs to make explicit populate/clear
  calls as it's all handled by rebind_subsystems(), and it gets proper
  error handling automatically.

* cgroup_mount() has been updated such that the root dentry and cgroup
  are linked before rebind_subsystems().  Also, the init_cred dancing
  and base file handling are moved right above rebind_subsystems()
  call and proper error handling for the base files is added.  While
  at it, add a comment explaining what's going on with the cred thing.

* cgroup_kill_sb() calls rebind_subsystems() to unbind all subsystems
  which now implies removing all subsystem files which requires the
  directory's i_mutex.  Grab it.  This means that files on the root
  cgroup are removed earlier - they used to be deleted from generic
  super_block cleanup from vfs.  This doesn't lead to any functional
  difference and it's cleaner to do the clean up explicitly for all
  files.

Combined with the previous changes, this makes all cgroup file
creation errors handled correctly.

v2: Added comment on init_cred.

v3: Li spotted that cgroup_mount() wasn't freeing tmp_links after base
    file addition failure.  Fix it by adding free_tmp_links error
    handling label.

v4: v3 introduced build bugs which got noticed by Fengguang's awesome
    kbuild test robot.  Fixed, and shame on me.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NLi Zefan <lizefan@huawei.com>
Cc: Fengguang Wu <fengguang.wu@intel.com>

rebind_subsystems() will be updated to handle file creations and
removals with proper error handling and to do that will need to
perform file operations before actually adding the subsystem to the
hierarchy.

To enable such usage, update cgroup_populate/clear_dir() to use
for_each_subsys() instead of for_each_root_subsys() so that they
operate on all subsystems specified by @subsys_mask whether that
subsystem is currently bound to the hierarchy or not.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NLi Zefan <lizefan@huawei.com>

cgroup_populate_dir() didn't use to check whether the actual file
creations were successful and could return success with only subset of
the requested files created, which is nasty.

This patch udpates cgroup_populate_dir() so that it either succeeds
with all files or fails with no file.

v2: The original patch also converted for_each_root_subsys() usages to
    for_each_subsys() without explaining why.  That part has been
    moved to a separate patch.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NLi Zefan <lizefan@huawei.com>

cgroup_populate/clear_dir() currently take @base_files and adds and
removes, respectively, cgroup_base_files[] to the directory.  File
additions and removals are being reorganized for proper error handling
and more dynamic handling for the unified hierarchy, and mixing base
and subsys file handling into the same functions gets a bit confusing.

This patch moves base file handling out of cgroup_populate/clear_dir()
into their users - cgroup_mount(), cgroup_create() and
cgroup_destroy_locked().

Note that this changes the behavior of base file removal.  If
@base_files is %true, cgroup_clear_dir() used to delete files
regardless of cftype until there's no files left.  Now, only files
with matching cfts are removed.  As files can only be created by the
base or registered cftypes, this shouldn't result in any behavior
difference.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NLi Zefan <lizefan@huawei.com>

cgroup_add_cftypes() uses cgroup_cfts_commit() to actually create the
files; however, both functions ignore actual file creation errors and
just assume success.  This can lead to, for example, blkio hierarchy
with some of the cgroups with only subset of interface files populated
after cfq-iosched is loaded under heavy memory pressure, which is
nasty.

This patch updates cgroup_cfts_commit() and cgroup_add_cftypes() to
guarantee that all files are created on success and no file is created
on failure.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NLi Zefan <lizefan@huawei.com>

cgroup_addrm_files() mishandled error return value from
cgroup_add_file() and returns error iff the last file fails to create.
As we're in the process of cleaning up file add/rm error handling and
will reliably propagate file creation failures, there's no point in
keeping adding files after a failure.

Replace the broken error collection logic with immediate error return.
While at it, add lockdep assertions and function comment.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NLi Zefan <lizefan@huawei.com>

* Rename it to cgroup_clear_dir() and make it take the pointer to the
  target cgroup instead of the the dentry.  This makes the function
  consistent with its counterpart - cgroup_populate_dir().

* Move cgroup_clear_directory() invocation from cgroup_d_remove_dir()
  to cgroup_remount() so that the function doesn't have to determine
  the cgroup pointer back from the dentry.  cgroup_d_remove_dir() now
  only deals with vfs, which is slightly cleaner.

This patch doesn't introduce any functional differences.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NLi Zefan <lizefan@huawei.com>

0ce6cba3 ("cgroup: CGRP_ROOT_SUBSYS_BOUND should be ignored when
comparing mount options") only updated the remount path but
CGRP_ROOT_SUBSYS_BOUND should also be ignored when comparing options
while mounting an existing hierarchy.  As option mismatch triggers a
warning but doesn't fail the mount without sane_behavior, this only
triggers a spurious warning message.

Fix it by only comparing CGRP_ROOT_OPTION_MASK bits when comparing new
and existing root options.
Signed-off-by: NTejun Heo <tj@kernel.org>

1672d040 ("cgroup: fix cgroupfs_root early destruction path")
introduced CGRP_ROOT_SUBSYS_BOUND which is used to mark completion of
subsys binding on a new root; however, this broke remounts.
cgroup_remount() doesn't allow changing root options via remount and
CGRP_ROOT_SUBSYS_BOUND, which is set on all fully initialized roots,
makes the function reject all remounts.

Fix it by putting the options part in the lower 16 bits of root->flags
and masking the comparions.  While at it, make cgroup_remount() emit
an error message explaining why it's rejecting a remount request, so
that it's less of a mystery.
Signed-off-by: NTejun Heo <tj@kernel.org>

eb178d06 ("cgroup: grab cgroup_mutex in
drop_parsed_module_refcounts()") made drop_parsed_module_refcounts()
grab cgroup_mutex to make lockdep assertion in for_each_subsys()
happy.  Unfortunately, cgroup_remount() calls the function while
holding cgroup_mutex in its failure path leading to the following
deadlock.

# mount -t cgroup -o remount,memory,blkio cgroup blkio

 cgroup: option changes via remount are deprecated (pid=525 comm=mount)

 =============================================
 [ INFO: possible recursive locking detected ]
 3.10.0-rc4-work+ #1 Not tainted
 ---------------------------------------------
 mount/525 is trying to acquire lock:
  (cgroup_mutex){+.+.+.}, at: [<ffffffff8110a3e1>] drop_parsed_module_refcounts+0x21/0xb0

 but task is already holding lock:
  (cgroup_mutex){+.+.+.}, at: [<ffffffff8110e4e1>] cgroup_remount+0x51/0x200

 other info that might help us debug this:
  Possible unsafe locking scenario:

	CPU0
	----
   lock(cgroup_mutex);
   lock(cgroup_mutex);

  *** DEADLOCK ***

  May be due to missing lock nesting notation

 4 locks held by mount/525:
  #0:  (&type->s_umount_key#30){+.+...}, at: [<ffffffff811e9a0d>] do_mount+0x2bd/0xa30
  #1:  (&sb->s_type->i_mutex_key#9){+.+.+.}, at: [<ffffffff8110e4d3>] cgroup_remount+0x43/0x200
  #2:  (cgroup_mutex){+.+.+.}, at: [<ffffffff8110e4e1>] cgroup_remount+0x51/0x200
  #3:  (cgroup_root_mutex){+.+.+.}, at: [<ffffffff8110e4ef>] cgroup_remount+0x5f/0x200

 stack backtrace:
 CPU: 2 PID: 525 Comm: mount Not tainted 3.10.0-rc4-work+ #1
 Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
  ffffffff829651f0 ffff88000ec2fc28 ffffffff81c24bb1 ffff88000ec2fce8
  ffffffff810f420d 0000000000000006 0000000000000001 0000000000000056
  ffff8800153b4640 ffff880000000000 ffffffff81c2e468 ffff8800153b4640
 Call Trace:
  [<ffffffff81c24bb1>] dump_stack+0x19/0x1b
  [<ffffffff810f420d>] __lock_acquire+0x15dd/0x1e60
  [<ffffffff810f531c>] lock_acquire+0x9c/0x1f0
  [<ffffffff81c2a805>] mutex_lock_nested+0x65/0x410
  [<ffffffff8110a3e1>] drop_parsed_module_refcounts+0x21/0xb0
  [<ffffffff8110e63e>] cgroup_remount+0x1ae/0x200
  [<ffffffff811c9bb2>] do_remount_sb+0x82/0x190
  [<ffffffff811e9d41>] do_mount+0x5f1/0xa30
  [<ffffffff811ea203>] SyS_mount+0x83/0xc0
  [<ffffffff81c2fb82>] system_call_fastpath+0x16/0x1b

Fix it by moving the drop_parsed_module_refcounts() invocation outside
cgroup_mutex.
Signed-off-by: NTejun Heo <tj@kernel.org>

kernel/cgroup.c still has places where a RCU pointer is set and
accessed directly without going through RCU_INIT_POINTER() or
rcu_dereference_protected().  They're all properly protected accesses
so nothing is broken but it leads to spurious sparse RCU address space
warnings.

Substitute direct accesses with RCU_INIT_POINTER() and
rcu_dereference_protected().  Note that %true is specified as the
extra condition for all derference updates.  This isn't ideal as all
it does is suppressing warning without actually policing
synchronization rules; however, most are scheduled to be removed
pretty soon along with css_id itself, so no reason to be more
elaborate.

Combined with the previous changes, this removes all RCU related
sparse warnings from cgroup.
Signed-off-by: NTejun Heo <tj@kernel.org>
Reported-by: NFengguang Wu <fengguang.wu@intel.com>
Acked-by; Li Zefan <lizefan@huawei.com>

There are several places in kernel/cgroup.c where task->cgroups is
accessed and modified without going through proper RCU accessors.
None is broken as they're all lock protected accesses; however, this
still triggers sparse RCU address space warnings.

* Consistently use task_css_set() for task->cgroups dereferencing.

* Use RCU_INIT_POINTER() to clear task->cgroups to &init_css_set on
  exit.

* Remove unnecessary rcu_dereference_raw() from cset->subsys[]
  dereference in cgroup_exit().
Signed-off-by: NTejun Heo <tj@kernel.org>
Reported-by: NFengguang Wu <fengguang.wu@intel.com>
Acked-by: NLi Zefan <lizefan@huawei.com>

This isn't strictly necessary as all subsystems specified in
@subsys_mask are guaranteed to be pinned; however, it does spuriously
trigger lockdep warning.  Let's grab cgroup_mutex around it.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NLi Zefan <lizefan@huawei.com>

cgroupfs_root used to have ->actual_subsys_mask in addition to
->subsys_mask.  a8a648c4 ("cgroup: remove
cgroup->actual_subsys_mask") removed it noting that the subsys_mask is
essentially temporary and doesn't belong in cgroupfs_root; however,
the patch made it impossible to tell whether a cgroupfs_root actually
has the subsystems bound or just have the bits set leading to the
following BUG when trying to mount with subsystems which are already
mounted elsewhere.

 kernel BUG at kernel/cgroup.c:1038!
 invalid opcode: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC
 ...
 CPU: 1 PID: 7973 Comm: mount Tainted: G        W    3.10.0-rc7-next-20130625-sasha-00011-g1c1dc0e #1105
 task: ffff880fc0ae8000 ti: ffff880fc0b9a000 task.ti: ffff880fc0b9a000
 RIP: 0010:[<ffffffff81249b29>]  [<ffffffff81249b29>] rebind_subsystems+0x409/0x5f0
 ...
 Call Trace:
  [<ffffffff8124bd4f>] cgroup_kill_sb+0xff/0x210
  [<ffffffff813d21af>] deactivate_locked_super+0x4f/0x90
  [<ffffffff8124f3b3>] cgroup_mount+0x673/0x6e0
  [<ffffffff81257169>] cpuset_mount+0xd9/0x110
  [<ffffffff813d2580>] mount_fs+0xb0/0x2d0
  [<ffffffff81404afd>] vfs_kern_mount+0xbd/0x180
  [<ffffffff814070b5>] do_new_mount+0x145/0x2c0
  [<ffffffff814085d6>] do_mount+0x356/0x3c0
  [<ffffffff8140873d>] SyS_mount+0xfd/0x140
  [<ffffffff854eb600>] tracesys+0xdd/0xe2

We still want rebind_subsystems() to take added/removed masks, so
let's fix it by marking whether a cgroupfs_root has finished binding
or not.  Also, document what's going on around ->subsys_mask
initialization so that similar mistakes aren't repeated.
Signed-off-by: NTejun Heo <tj@kernel.org>
Reported-by: NSasha Levin <sasha.levin@oracle.com>
Acked-by: NLi Zefan <lizefan@huawei.com>

Before 1a574231 ("cgroup: make hierarchy_id use cyclic idr"),
hierarchy IDs were allocated from 0.  As the dummy hierarchy was
always the one first initialized, it got assigned 0 and all other
hierarchies from 1.  The patch accidentally changed the minimum
useable ID to 2.

Let's restore ID 0 for dummy_root and while at it reserve 1 for
unified hierarchy.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NLi Zefan <lizefan@huawei.com>
Cc: stable@vger.kernel.org

There are quite a few places where all loaded [builtin] subsys are
iterated.  Implement for_each_[builtin_]subsys() and replace manual
iterations with those to simplify those places a bit.  The new
iterators automatically skip NULL subsystems.  This shouldn't cause
any functional difference.

Iteration loops which scan all subsystems and then skipping modular
ones explicitly are converted to use for_each_builtin_subsys().

While at it, reorder variable declarations and adjust whitespaces a
bit in the affected functions.

v2: Add lockdep_assert_held() in for_each_subsys() and add comments
    about synchronization as suggested by Li.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NLi Zefan <lizefan@huawei.com>

cgroup_init() was doing init_css_set initialization outside
cgroup_mutex, which is fine but we want to add lockdep annotation on
subsystem iterations and cgroup_init() will trigger it spuriously.
Move init_css_set initialization inside cgroup_mutex.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NLi Zefan <lizefan@huawei.com>

for_each_subsys() walks over subsystems attached to a hierarchy and
we're gonna add iterators which walk over all available subsystems.
Rename for_each_subsys() to for_each_root_subsys() so that it's more
appropriately named and for_each_subsys() can be used to iterate all
subsystems.

While at it, remove unnecessary underbar prefix from macro arguments,
put them inside parentheses, and adjust indentation for the two
for_each_*() macros.

This patch is purely cosmetic.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NLi Zefan <lizefan@huawei.com>

find_css_set() passes uninitialized on-stack template[] array to
find_existing_css_set() which sets the entries for all subsystems.
Passing around an uninitialized array is a bit icky and we want to
introduce an iterator which only iterates loaded subsystems.  Let's
initialize it on definition.

While at it, also make the following cosmetic cleanups.

* Convert to proper /** comments.

* Reorder variable declarations.

* Replace comment on synchronization with lockdep_assert_held().

This patch doesn't make any functional differences.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NLi Zefan <lizefan@huawei.com>

cgroup curiously has two subsystem masks, ->subsys_mask and
->actual_subsys_mask.  The latter only exists because the new target
subsys_mask is passed into rebind_subsystems() via @root>subsys_mask.
rebind_subsystems() needs to know what the current mask is to decide
how to reach the target mask so ->actual_subsys_mask is used as the
temp location to remember the current state.

Adding a temporary field to a permanent data structure is rather silly
and can be misleading.  Update rebind_subsystems() to take @added_mask
and @removed_mask instead and remove @root->actual_subsys_mask.

This patch shouldn't introduce any behavior changes.

v2: Comment and description updated as suggested by Li.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NLi Zefan <lizefan@huawei.com>

Global variable names in kernel/cgroup.c are asking for trouble -
subsys, roots, rootnode and so on.  Rename them to have "cgroup_"
prefix.

* s/subsys/cgroup_subsys/

* s/rootnode/cgroup_dummy_root/

* s/dummytop/cgroup_cummy_top/

* s/roots/cgroup_roots/

* s/root_count/cgroup_root_count/

This patch is purely cosmetic.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NLi Zefan <lizefan@huawei.com>

Cont is short for container. control group was named process container
at first, but then people found container already has a meaning in
linux kernel.

Clean up the leftover variable name @cont.
Signed-off-by: NLi Zefan <lizefan@huawei.com>
Signed-off-by: NTejun Heo <tj@kernel.org>

cgroup_serial_nr_cursor was created atomic64_t because I thought it
was never gonna used for anything other than assigning unique numbers
to cgroups and didn't want to worry about synchronization; however,
now we're using it as an event-stamp to distinguish cgroups created
before and after certain point which assumes that it's protected by
cgroup_mutex.

Let's make it clear by making it a u64.  Also, rename it to
cgroup_serial_nr_next and make it point to the next nr to allocate so
that where it's pointing to is clear and more conventional.
Signed-off-by: NTejun Heo <tj@kernel.org>
Cc: Li Zefan <lizefan@huawei.com>

We used root->allcg_list to iterate cgroup hierarchy because at that time
cgroup_for_each_descendant_pre() hasn't been invented.

tj: In cgroup_cfts_commit(), s/@serial_nr/@update_upto/, move the
    assignment right above releasing cgroup_mutex and explain what's
    going on there.
Signed-off-by: NLi Zefan <lizefan@huawei.com>
Signed-off-by: NTejun Heo <tj@kernel.org>

The next patch will use it to determine if a cgroup is newly created
while we're iterating the cgroup hierarchy.

tj: Rephrased the comment on top of cgroup_serial_nr_cursor.
Signed-off-by: NLi Zefan <lizefan@huawei.com>
Signed-off-by: NTejun Heo <tj@kernel.org>

The memory allocated in cgroup_add_cftypes() should be freed. The
effect of this bug is we leak a bit memory everytime we unload
cfq-iosched module if blkio cgroup is enabled.
Signed-off-by: NLi Zefan <lizefan@huawei.com>
Signed-off-by: NTejun Heo <tj@kernel.org>

 commit 5db9a4d9
 Author: Tejun Heo <tj@kernel.org>
 Date:   Sat Jul 7 16:08:18 2012 -0700

     cgroup: fix cgroup hierarchy umount race

This commit fixed a race caused by the dput() in css_dput_fn(), but
the dput() in cgroup_event_remove() can also lead to the same BUG().
Signed-off-by: NLi Zefan <lizefan@huawei.com>
Signed-off-by: NTejun Heo <tj@kernel.org>
Cc: stable@vger.kernel.org

cgroup_cfts_commit() uses dget() to keep cgroup alive after cgroup_mutex
is dropped, but dget() won't prevent cgroupfs from being umounted. When
the race happens, vfs will see some dentries with non-zero refcnt while
umount is in process.

Keep running this:
  mount -t cgroup -o blkio xxx /cgroup
  umount /cgroup

And this:
  modprobe cfq-iosched
  rmmod cfs-iosched

After a while, the BUG() in shrink_dcache_for_umount_subtree() may
be triggered:

  BUG: Dentry xxx{i=0,n=blkio.yyy} still in use (1) [umount of cgroup cgroup]
Signed-off-by: NLi Zefan <lizefan@huawei.com>
Signed-off-by: NTejun Heo <tj@kernel.org>
Cc: stable@vger.kernel.org

cgroup's rename(2) isn't a proper migration implementation - it can't
move the cgroup to a different parent in the hierarchy.  All it can do
is swapping the name string for that cgroup.  This isn't useful and
can mislead users to think that cgroup supports proper cgroup-level
migration.  Disallow rename(2) if sane_behavior.

v2: Fail with -EPERM instead of -EINVAL so that it matches the vfs
    return value when ->rename is not implemented as suggested by Li.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NLi Zefan <lizefan@huawei.com>

A css (cgroup_subsys_state) is how each cgroup is represented to a
controller.  As such, it can be used in hot paths across the various
subsystems different controllers are associated with.

One of the common operations is reference counting, which up until now
has been implemented using a global atomic counter and can have
significant adverse impact on scalability.  For example, css refcnt
can be gotten and put multiple times by blkcg for each IO request.
For highops configurations which try to do as much per-cpu as
possible, the global frequent refcnting can be very expensive.

In general, given the various and hugely diverse paths css's end up
being used from, we need to make it cheap and highly scalable.  In its
usage, css refcnting isn't very different from module refcnting.

This patch converts css refcnting to use the recently added
percpu_ref.  css_get/tryget/put() directly maps to the matching
percpu_ref operations and the deactivation logic is no longer
necessary as percpu_ref already has refcnt killing.

The only complication is that as the refcnt is per-cpu,
percpu_ref_kill() in itself doesn't ensure that further tryget
operations will fail, which we need to guarantee before invoking
->css_offline()'s.  This is resolved collecting kill confirmation
using percpu_ref_kill_and_confirm() and initiating the offline phase
of destruction after all css refcnt's are confirmed to be seen as
killed on all CPUs.  The previous patches already splitted destruction
into two phases, so percpu_ref_kill_and_confirm() can be hooked up
easily.

This patch removes css_refcnt() which is used for rcu dereference
sanity check in css_id().  While we can add a percpu refcnt API to ask
the same question, css_id() itself is scheduled to be removed fairly
soon, so let's not bother with it.  Just drop the sanity check and use
rcu_dereference_raw() instead.

v2: - init_cgroup_css() was calling percpu_ref_init() without checking
      the return value.  This causes two problems - the obvious lack
      of error handling and percpu_ref_init() being called from
      cgroup_init_subsys() before the allocators are up, which
      triggers warnings but doesn't cause actual problems as the
      refcnt isn't used for roots anyway.  Fix both by moving
      percpu_ref_init() to cgroup_create().

    - The base references were put too early by
      percpu_ref_kill_and_confirm() and cgroup_offline_fn() put the
      refs one extra time.  This wasn't noticeable because css's go
      through another RCU grace period before being freed.  Update
      cgroup_destroy_locked() to grab an extra reference before
      killing the refcnts.  This problem was noticed by Kent.
Signed-off-by: NTejun Heo <tj@kernel.org>
Reviewed-by: NKent Overstreet <koverstreet@google.com>
Acked-by: NLi Zefan <lizefan@huawei.com>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Mike Snitzer <snitzer@redhat.com>
Cc: Vivek Goyal <vgoyal@redhat.com>
Cc: "Alasdair G. Kergon" <agk@redhat.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Mikulas Patocka <mpatocka@redhat.com>
Cc: Glauber Costa <glommer@gmail.com>

Split cgroup_destroy_locked() into two steps and put the latter half
into cgroup_offline_fn() which is executed from a work item.  The
latter half is responsible for offlining the css's, removing the
cgroup from internal lists, and propagating release notification to
the parent.  The separation is to allow using percpu refcnt for css.

Note that this allows for other cgroup operations to happen between
the first and second halves of destruction, including creating a new
cgroup with the same name.  As the target cgroup is marked DEAD in the
first half and cgroup internals don't care about the names of cgroups,
this should be fine.  A comment explaining this will be added by the
next patch which implements the actual percpu refcnting.

As RCU freeing is guaranteed to happen after the second step of
destruction, we can use the same work item for both.  This patch
renames cgroup->free_work to ->destroy_work and uses it for both
purposes.  INIT_WORK() is now performed right before queueing the work
item.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NLi Zefan <lizefan@huawei.com>

This patch reorders the operations in cgroup_destroy_locked() such
that the userland visible parts happen before css offlining and
removal from the ->sibling list.  This will be used to make css use
percpu refcnt.

While at it, split out CGRP_DEAD related comment from the refcnt
deactivation one and correct / clarify how different guarantees are
met.

While this patch changes the specific order of operations, it
shouldn't cause any noticeable behavior difference.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NLi Zefan <lizefan@huawei.com>

cgroup->count tracks the number of css_sets associated with the cgroup
and used only to verify that no css_set is associated when the cgroup
is being destroyed.  It's superflous as the destruction path can
simply check whether cgroup->cset_links is empty instead.

Drop cgroup->count and check ->cset_links directly from
cgroup_destroy_locked().
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NLi Zefan <lizefan@huawei.com>