cgroup in general is moving towards using cgroup_subsys_state as the
fundamental structural component and css_parent() was introduced to
convert from using cgroup->parent to css->parent.  It was quite some
time ago and we're moving forward with making css more prominent.

This patch drops the trivial wrapper css_parent() and let the users
dereference css->parent.  While at it, explicitly mark fields of css
which are public and immutable.

v2: New usage from device_cgroup.c converted.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Michal Hocko <mhocko@suse.cz>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Acked-by: "David S. Miller" <davem@davemloft.net>
Acked-by: Li Zefan <lizefan@huawei.com>
Cc: Vivek Goyal <vgoyal@redhat.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Johannes Weiner <hannes@cmpxchg.org>

9395a450 ("cgroup: enable refcnting for root csses") enabled
reference counting for root csses (cgroup_subsys_states) so that
cgroup's self csses can be used to manage the lifetime of the
containing cgroups.

Unfortunately, this change was incorrect.  During early init,
cgrp_dfl_root self css refcnt is used.  percpu_ref can't initialized
during early init and its initialization is deferred till
cgroup_init() time.  This means that cpu was using percpu_ref which
wasn't properly initialized.  Due to the way percpu variables are laid
out on x86, this didn't blow up immediately on x86 but ended up
incrementing and decrementing the percpu variable at offset zero,
whatever it may be; however, on other archs, this caused fault and
early boot failure.

As cgroup self csses for root cgroups of non-dfl hierarchies need
working refcounting, we can't revert 9395a450.  This patch adds
CSS_NO_REF which explicitly inhibits reference counting on the css and
sets it on all normal (non-self) csses and cgroup_dfl_root self css.

v2: cgrp_dfl_root.self is the offending one.  Set the flag on it.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Stephen Warren <swarren@nvidia.com>
Tested-by: Stephen Warren <swarren@nvidia.com>
Fixes: 9395a450 ("cgroup: enable refcnting for root csses")

Currently cgroup implements refcnting separately using atomic_t
cgroup->refcnt.  The destruction paths of cgroup and css are rather
complex and bear a lot of similiarities including the use of RCU and
bouncing to a work item.

This patch makes cgroup use the refcnt of self css for refcnting
instead of using its own.  This makes cgroup refcnting use css's
percpu refcnt and share the destruction mechanism.

* css_release_work_fn() and css_free_work_fn() are updated to handle
  both csses and cgroups.  This is a bit messy but should do until we
  can make cgroup->self a full css, which currently can't be done
  thanks to multiple hierarchies.

* cgroup_destroy_locked() now performs
  percpu_ref_kill(&cgrp->self.refcnt) instead of cgroup_put(cgrp).

* Negative refcnt sanity check in cgroup_get() is no longer necessary
  as percpu_ref already handles it.

* Similarly, as a cgroup which hasn't been killed will never be
  released regardless of its refcnt value and percpu_ref has sanity
  check on kill, cgroup_is_dead() sanity check in cgroup_put() is no
  longer necessary.

* As whether a refcnt reached zero or not can only be decided after
  the reference count is killed, cgroup_root->cgrp's refcnting can no
  longer be used to decide whether to kill the root or not.  Let's
  make cgroup_kill_sb() explicitly initiate destruction if the root
  doesn't have any children.  This makes sense anyway as unmounted
  cgroup hierarchy without any children should be destroyed.

While this is a bit messy, this will allow pushing more bookkeeping
towards cgroup->self and thus handling cgroups and csses in more
uniform way.  In the very long term, it should be possible to
introduce a base subsystem and convert the self css to a proper one
making things whole lot simpler and unified.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>

Currently, css_get(), css_tryget() and css_tryget_online() are noops
for root csses as an optimization; however, we're planning to use css
refcnts to track of cgroup lifetime too and root cgroups also need to
be reference counted.  Since css has been converted to percpu_refcnt,
the overhead of refcnting is miniscule and this optimization isn't too
meaningful anymore.  Furthermore, controllers which optimize the root
cgroup often never even invoke these functions in their hot paths.

This patch enables refcnting for root csses too.  This makes CSS_ROOT
flag unused and removes it.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>

css release is planned to do more and would require process context.
Bounce it through css->destroy_work.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>

cgroup_destroy_css_killed() is cgroup destruction stage which happens
after all csses are offlined.  After the recent updates, it no longer
does anything other than putting the base reference.  This patch
removes the function and makes cgroup_destroy_locked() put the base
ref at the end isntead.

This also makes cgroup->nr_css unnecessary.  Removed.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>

Move cgroup->sibling unlinking from cgroup_destroy_css_killed() to
cgroup_put().  This is later but still before the RCU grace period, so
it doesn't break css_next_child() although there now is a larger
window in which a dead cgroup is visible during css iteration.  As css
iteration always could have included offline csses, this doesn't
affect correctness; however, it does make css_next_child() fall back
to reiterting mode more often.  This also makes cgroup_put() directly
take cgroup_mutex, which limits where it can be called from.  These
are not immediately problematic and will be dealt with later.

This change enables simplification of cgroup destruction path.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>

Currently, check_for_release() on the parent of a destroyed cgroup is
invoked from cgroup_destroy_css_killed().  This is because this is
where the destroyed cgroup can be removed from the parent's children
list.  check_for_release() tests the emptiness of the list directly,
so invoking it before removing the cgroup from the list makes it think
that the parent still has children even when it no longer does.

This patch updates check_for_release() to use
cgroup_has_live_children() instead of directly testing ->children
emptiness and moves check_for_release(parent) earlier to the end of
cgroup_destroy_locked().  As cgroup_has_live_children() ignores
cgroups marked DEAD, check_for_release() functions correctly as long
as it's called after asserting DEAD.

This makes release notification slightly more timely and more
importantly enables further simplification of cgroup destruction path.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>

We're expecting another user.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>

cgroup->dummy_css is used as the placeholder css when performing css
oriended operations on the cgroup.  We're gonna shift more cgroup
management to this css.  Let's rename it to ->self and move it to the
top.

This is pure rename and field relocation.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>

cgroup_mount() uses dumb delay-and-retry logic to wait for cgroup_root
which is being destroyed.  The retry currently loops inside
cgroup_mount() proper.  This patch makes it return with
restart_syscall() instead so that retry travels out to userland
boundary.

This slightly simplifies the logic and more importantly makes the
retry logic behave better when the wait for some reason becomes
lengthy or infinite by allowing the operation to be suspended or
terminated from userland.

v2: The original patch forgot to free memory allocated for @opts.
    Fixed.  Caught by Li Zefan.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>

cgroup_tree_mutex was introduced to work around the circular
dependency between cgroup_mutex and kernfs active protection - some
kernfs file and directory operations needed cgroup_mutex putting
cgroup_mutex under active protection but cgroup also needs to be able
to access cgroup hierarchies and cftypes to determine which
kernfs_nodes need to be removed.  cgroup_tree_mutex nested above both
cgroup_mutex and kernfs active protection and used to protect the
hierarchy and cftypes.  While this worked, it added a lot of double
lockings and was generally cumbersome.

kernfs provides a mechanism to opt out of active protection and cgroup
was already using it for removal and subtree_control.  There's no
reason to mix both methods of avoiding circular locking dependency and
the preceding cgroup_kn_lock_live() changes applied it to all relevant
cgroup kernfs operations making it unnecessary to nest cgroup_mutex
under kernfs active protection.  The previous patch reversed the
original lock ordering and put cgroup_mutex above kernfs active
protection.

After these changes, all cgroup_tree_mutex usages are now accompanied
by cgroup_mutex making the former completely redundant.  This patch
removes cgroup_tree_mutex and all its usages.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>

After the recent cgroup_kn_lock_live() changes, cgroup_mutex is no
longer nested below kernfs active protection.  The two don't have any
relationship now.

This patch nests kernfs active protection under cgroup_mutex.  All
cftype operations now require both cgroup_tree_mutex and cgroup_mutex,
temporary cgroup_mutex releases over kernfs operations are removed,
and cgroup_add/rm_cftypes() grab both mutexes.

This makes cgroup_tree_mutex redundant, which will be removed by the
next patch.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>

Make __cgroup_procs_write() and cgroup_release_agent_write() use
cgroup_kn_lock_live() and cgroup_kn_unlock() instead of
cgroup_lock_live_group().  This puts the operations under both
cgroup_tree_mutex and cgroup_mutex protection without circular
dependency from kernfs active protection.  Also, this means that
cgroup_mutex is no longer nested below kernfs active protection.
There is no longer any place where the two locks interact.

This leaves cgroup_lock_live_group() without any user.  Removed.

This will help simplifying cgroup locking.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>

cgroup_mkdir(), cgroup_rmdir() and cgroup_subtree_control_write()
share the logic to break active protection so that they can grab
cgroup_tree_mutex which nests above active protection and/or remove
self.  Factor out this logic into cgroup_kn_lock_live() and
cgroup_kn_unlock().

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

The ->priv field of a cgroup directory kernfs_node points back to the
cgroup.  This field is RCU cleared in cgroup_destroy_locked() for
non-kernfs accesses from css_tryget_from_dir() and
cgroupstats_build().

As these are only applicable to cgroups which finished creation
successfully and fully initialized cgroups are always removed by
cgroup_rmdir(), this can be safely moved to the end of cgroup_rmdir().

This will help simplifying cgroup locking and shouldn't introduce any
behavior difference.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>

Move cgroup_lock_live_group() invocation upwards to right below
cgroup_tree_mutex in cgroup_subtree_control_write().  This is to help
the planned locking simplification.

This doesn't make any userland-visible behavioral changes.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>

cgroup_mkdir() is the sole user of cgroup_create().  Let's collapse
the latter into the former.  This will help simplifying locking.
While at it, remove now stale comment about inode locking.

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

Reorganize cgroup_create() so that all paths share unlock out path.

* All err_* labels are renamed to out_* as they're now shared by both
  success and failure paths.

* @err renamed to @ret for the similar reason as above and so that
  it's more consistent with other functions.

* cgroup memory allocation moved after locking so that freeing failed
  cgroup happens before unlocking.  While this moves more code inside
  critical section, memory allocations inside cgroup locking are
  already pretty common and this is unlikely to make any noticeable
  difference.

* While at it, replace a stray @parent->root dereference with @root.

This reorganization will help simplifying locking.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>

Now that cgroup_subtree_control_write() has access to the associated
kernfs_open_file and thus the kernfs_node, there's no need to cache it
in cgroup->control_kn on creation.  Remove cgroup->control_kn and use
@of->kn directly.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>

cgroup_tasks_write() and cgroup_procs_write() are currently using
cftype->write_u64().  This patch converts them to use cftype->write()
instead.  This allows access to the associated kernfs_open_file which
will be necessary to implement the planned kernfs active protection
manipulation for these files.

This shifts buffer parsing to attach_task_by_pid() and makes it return
@nbytes on success.  Let's rename it to __cgroup_procs_write() to
clearly indicate that this is a write handler implementation.

This patch doesn't introduce any visible behavior changes.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>

cftype->trigger() is pointless.  It's trivial to ignore the input
buffer from a regular ->write() operation.  Convert all ->trigger()
users to ->write() and remove ->trigger().

This patch doesn't introduce any visible behavior changes.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.cz>

Convert all cftype->write_string() users to the new cftype->write()
which maps directly to kernfs write operation and has full access to
kernfs and cgroup contexts.  The conversions are mostly mechanical.

* @css and @cft are accessed using of_css() and of_cft() accessors
  respectively instead of being specified as arguments.

* Should return @nbytes on success instead of 0.

* @buf is not trimmed automatically.  Trim if necessary.  Note that
  blkcg and netprio don't need this as the parsers already handle
  whitespaces.

cftype->write_string() has no user left after the conversions and
removed.

While at it, remove unnecessary local variable @p in
cgroup_subtree_control_write() and stale comment about
CGROUP_LOCAL_BUFFER_SIZE in cgroup_freezer.c.

This patch doesn't introduce any visible behavior changes.

v2: netprio was missing from conversion.  Converted.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Aristeu Rozanski <arozansk@redhat.com>
Acked-by: Vivek Goyal <vgoyal@redhat.com>
Acked-by: Li Zefan <lizefan@huawei.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Neil Horman <nhorman@tuxdriver.com>
Cc: "David S. Miller" <davem@davemloft.net>

During the recent conversion to kernfs, cftype's seq_file operations
are updated so that they are directly mapped to kernfs operations and
thus can fully access the associated kernfs and cgroup contexts;
however, write path hasn't seen similar updates and none of the
existing write operations has access to, for example, the associated
kernfs_open_file.

Let's introduce a new operation cftype->write() which maps directly to
the kernfs write operation and has access to all the arguments and
contexts.  This will replace ->write_string() and ->trigger() and ease
manipulation of kernfs active protection from cgroup file operations.

Two accessors - of_cft() and of_css() - are introduced to enable
accessing the associated cgroup context from cftype->write() which
only takes kernfs_open_file for the context information.  The
accessors for seq_file operations - seq_cft() and seq_css() - are
rewritten to wrap the of_ accessors.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>

Unlike the more usual refcnting, what css_tryget() provides is the
distinction between online and offline csses instead of protection
against upping a refcnt which already reached zero.  cgroup is
planning to provide actual tryget which fails if the refcnt already
reached zero.  Let's rename the existing trygets so that they clearly
indicate that they're onliness.

I thought about keeping the existing names as-are and introducing new
names for the planned actual tryget; however, given that each
controller participates in the synchronization of the online state, it
seems worthwhile to make it explicit that these functions are about
on/offline state.

Rename css_tryget() to css_tryget_online() and css_tryget_from_dir()
to css_tryget_online_from_dir().  This is pure rename.

v2: cgroup_freezer grew new usages of css_tryget().  Update
    accordingly.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Michal Hocko <mhocko@suse.cz>
Acked-by: Li Zefan <lizefan@huawei.com>
Cc: Vivek Goyal <vgoyal@redhat.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>

release_path is now protected by release_agent_path_lock to allow
accessing it without grabbing cgroup_mutex; however,
cgroup_release_agent_show() was still grabbing cgroup_mutex.  Let's
convert it to release_agent_path_lock so that we don't have to worry
about this one for the planned locking updates.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>

After waiting for a child to finish offline,
cgroup_subtree_control_write() jumps up to retry from after the input
parsing and active protection breaking.  This retry makes the
scheduled locking update - removal of cgroup_tree_mutex - more
difficult.  Let's simplify it by returning with restart_syscall() for
retries.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>

I was confused that strsep() was equivalent to strtok_r() in skipping
over consecutive delimiters.  strsep() just splits at the first
occurrence of one of the delimiters which makes the parsing very
inflexible, which makes allowing multiple whitespace chars as
delimters kinda moot.  Let's just be consistently strict and require
list of tokens separated by spaces.  This is what
Documentation/cgroups/unified-hierarchy.txt describes too.

Also, parsing may access beyond the end of the string if the string
ends with spaces or is zero-length.  Make sure it skips zero-length
tokens.  Note that this also ensures that the parser doesn't puke on
multiple consecutive spaces.

v2: Add zero-length token skipping.

v3: Added missing space after "==".  Spotted by Li.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>

c1a71504 ("cgroup: don't recycle cgroup id until all csses' have
been destroyed") made cgroup ID persist until a cgroup is released and
add cgroup->subsys[] clearing to css_release() so that css_from_id()
doesn't return a css which has already been released which happens
before cgroup release; however, the right change here was updating
offline_css() to clear cgroup->subsys[] which was done by e3297803
("cgroup: cgroup->subsys[] should be cleared after the css is
offlined") instead of clearing it from css_release().

We're now clearing cgroup->subsys[] twice.  This is okay for
traditional hierarchies as a css's lifetime is the same as its
cgroup's; however, this confuses unified hierarchy and turning on and
off a controller repeatedly using "cgroup.subtree_control" can lead to
an oops like the following which happens because cgroup->subsys[] is
incorrectly cleared asynchronously by css_release().

 BUG: unable to handle kernel NULL pointer dereference at 00000000000000 08
 IP: [<ffffffff81130c11>] kill_css+0x21/0x1c0
 PGD 1170d067 PUD f0ab067 PMD 0
 Oops: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC
 Modules linked in:
 CPU: 2 PID: 459 Comm: bash Not tainted 3.15.0-rc2-work+ #5
 Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
 task: ffff880009296710 ti: ffff88000e198000 task.ti: ffff88000e198000
 RIP: 0010:[<ffffffff81130c11>]  [<ffffffff81130c11>] kill_css+0x21/0x1c0
 RSP: 0018:ffff88000e199dc8  EFLAGS: 00010202
 RAX: 0000000000000001 RBX: 0000000000000000 RCX: 0000000000000001
 RDX: 0000000000000001 RSI: ffffffff8238a968 RDI: ffff880009296f98
 RBP: ffff88000e199de0 R08: 0000000000000001 R09: 02b0000000000000
 R10: 0000000000000000 R11: ffff880009296fc0 R12: 0000000000000001
 R13: ffff88000db6fc58 R14: 0000000000000001 R15: ffff8800139dcc00
 FS:  00007ff9160c5740(0000) GS:ffff88001fb00000(0000) knlGS:0000000000000000
 CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
 CR2: 0000000000000008 CR3: 0000000013947000 CR4: 00000000000006e0
 Stack:
  ffff88000e199de0 ffffffff82389160 0000000000000001 ffff88000e199e80
  ffffffff8113537f 0000000000000007 ffff88000e74af00 ffff88000e199e48
  ffff880009296710 ffff88000db6fc00 ffffffff8239c100 0000000000000002
 Call Trace:
  [<ffffffff8113537f>] cgroup_subtree_control_write+0x85f/0xa00
  [<ffffffff8112fd18>] cgroup_file_write+0x38/0x1d0
  [<ffffffff8126fc97>] kernfs_fop_write+0xe7/0x170
  [<ffffffff811f2ae6>] vfs_write+0xb6/0x1c0
  [<ffffffff811f35ad>] SyS_write+0x4d/0xc0
  [<ffffffff81d0acd2>] system_call_fastpath+0x16/0x1b
 Code: 5c 41 5d 41 5e 41 5f 5d c3 90 0f 1f 44 00 00 55 48 89 e5 41 54 53 48 89 fb 48 83 ec 08 8b 05 37 ad 29 01 85 c0 0f 85 df 00 00 00 <48> 8b 43 08 48 8b 3b be 01 00 00 00 8b 48 5c d3 e6 e8 49 ff ff
 RIP  [<ffffffff81130c11>] kill_css+0x21/0x1c0
  RSP <ffff88000e199dc8>
 CR2: 0000000000000008
 ---[ end trace e7aae1f877c4e1b4 ]---

Remove the unnecessary cgroup->subsys[] clearing from css_release().
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>

cgroup_idr_remove() can be invoked from bh leading to lockdep
detecting possible AA deadlock (IN_BH/ON_BH).  Make the lock bh-safe.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>

cgroup_subtree_control_write() waits for offline to complete
child-by-child before enabling a controller; however, it has a couple
bugs.

* It doesn't initialize the wait_queue_t.  This can lead to infinite
  hang on the following schedule() among other things.

* It forgets to pin the child before releasing cgroup_tree_mutex and
  performing schedule().  The child may already be gone by the time it
  wakes up and invokes finish_wait().  Pin the child being waited on.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>

Merge branch 'for-3.15-fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/cgroup into for-3.16

Pull to receive e37a06f1 ("cgroup: fix the retry path of
cgroup_mount()") to avoid unnecessary conflicts with planned
cgroup_tree_mutex removal and also to be able to remove the temp fix
added by 36c38fb7 ("blkcg: use trylock on blkcg_pol_mutex in
blkcg_reset_stats()") afterwards.
Signed-off-by: Tejun Heo <tj@kernel.org>

While updating cgroup_freezer locking, 68fafb77d827 ("cgroup_freezer:
replace freezer->lock with freezer_mutex") introduced a bug in
update_if_frozen() where it returns with rcu_read_lock() held.  Fix it
by adding rcu_read_unlock() before returning.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: kbuild test robot <fengguang.wu@intel.com>

Pull to receive percpu_ref_tryget[_live]() changes.  Planned cgroup
changes will make use of them.
Signed-off-by: Tejun Heo <tj@kernel.org>

After 96d365e0 ("cgroup: make css_set_lock a rwsem and rename it
to css_set_rwsem"), css task iterators requires sleepable context as
it may block on css_set_rwsem.  I missed that cgroup_freezer was
iterating tasks under IRQ-safe spinlock freezer->lock.  This leads to
errors like the following on freezer state reads and transitions.

  BUG: sleeping function called from invalid context at /work
 /os/work/kernel/locking/rwsem.c:20
  in_atomic(): 0, irqs_disabled(): 0, pid: 462, name: bash
  5 locks held by bash/462:
   #0:  (sb_writers#7){.+.+.+}, at: [<ffffffff811f0843>] vfs_write+0x1a3/0x1c0
   #1:  (&of->mutex){+.+.+.}, at: [<ffffffff8126d78b>] kernfs_fop_write+0xbb/0x170
   #2:  (s_active#70){.+.+.+}, at: [<ffffffff8126d793>] kernfs_fop_write+0xc3/0x170
   #3:  (freezer_mutex){+.+...}, at: [<ffffffff81135981>] freezer_write+0x61/0x1e0
   #4:  (rcu_read_lock){......}, at: [<ffffffff81135973>] freezer_write+0x53/0x1e0
  Preemption disabled at:[<ffffffff81104404>] console_unlock+0x1e4/0x460

  CPU: 3 PID: 462 Comm: bash Not tainted 3.15.0-rc1-work+ #10
  Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
   ffff88000916a6d0 ffff88000e0a3da0 ffffffff81cf8c96 0000000000000000
   ffff88000e0a3dc8 ffffffff810cf4f2 ffffffff82388040 ffff880013aaf740
   0000000000000002 ffff88000e0a3de8 ffffffff81d05974 0000000000000246
  Call Trace:
   [<ffffffff81cf8c96>] dump_stack+0x4e/0x7a
   [<ffffffff810cf4f2>] __might_sleep+0x162/0x260
   [<ffffffff81d05974>] down_read+0x24/0x60
   [<ffffffff81133e87>] css_task_iter_start+0x27/0x70
   [<ffffffff8113584d>] freezer_apply_state+0x5d/0x130
   [<ffffffff81135a16>] freezer_write+0xf6/0x1e0
   [<ffffffff8112eb88>] cgroup_file_write+0xd8/0x230
   [<ffffffff8126d7b7>] kernfs_fop_write+0xe7/0x170
   [<ffffffff811f0756>] vfs_write+0xb6/0x1c0
   [<ffffffff811f121d>] SyS_write+0x4d/0xc0
   [<ffffffff81d08292>] system_call_fastpath+0x16/0x1b

freezer->lock used to be used in hot paths but that time is long gone
and there's no reason for the lock to be IRQ-safe spinlock or even
per-cgroup.  In fact, given the fact that a cgroup may contain large
number of tasks, it's not a good idea to iterate over them while
holding IRQ-safe spinlock.

Let's simplify locking by replacing per-cgroup freezer->lock with
global freezer_mutex.  This also makes the comments explaining the
intricacies of policy inheritance and the locking around it as the
states are protected by a common mutex.

The conversion is mostly straight-forward.  The followings are worth
mentioning.

* freezer_css_online() no longer needs double locking.

* freezer_attach() now performs propagation simply while holding
  freezer_mutex.  update_if_frozen() race no longer exists and the
  comment is removed.

* freezer_fork() now tests whether the task is in root cgroup using
  the new task_css_is_root() without doing rcu_read_lock/unlock().  If
  not, it grabs freezer_mutex and performs the operation.

* freezer_read() and freezer_change_state() grab freezer_mutex across
  the whole operation and pin the css while iterating so that each
  descendant processing happens in sleepable context.

Fixes: 96d365e0 ("cgroup: make css_set_lock a rwsem and rename it to css_set_rwsem")
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>

Determining the css of a task usually requires RCU read lock as that's
the only thing which keeps the returned css accessible till its
reference is acquired; however, testing whether a task belongs to the
root can be performed without dereferencing the returned css by
comparing the returned pointer against the root one in init_css_set[]
which never changes.

Implement task_css_is_root() which can be invoked in any context.
This will be used by the scheduled cgroup_freezer change.

v2: cgroup no longer supports modular controllers.  No need to export
    init_css_set.  Pointed out by Li.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>

Implement percpu_ref_tryget() which fails if the refcnt already
reached zero.  Note that this is different from the recently renamed
percpu_ref_tryget_live() which fails if the refcnt has been killed and
is draining the remaining references.  percpu_ref_tryget() succeeds on
a killed refcnt as long as its current refcnt is above zero.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Kent Overstreet <kmo@daterainc.com>

percpu_ref_tryget() is different from the usual tryget semantics in
that it fails if the refcnt is in its dying stage even if the refcnt
hasn't reached zero yet.  We're about to introduce the more
conventional tryget and the current one has only one user.  Let's
rename it to percpu_ref_tryget_live() so that it explicitly signifies
the peculiarities of its semantics.

This is pure rename.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Kent Overstreet <kmo@daterainc.com>

This cgroup flag has never been used.  Only CGRP_ROOT_SANE_BEHAVIOR is
used.  Remove it.
Signed-off-by: Tejun Heo <tj@kernel.org>

Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Fabian Frederick <fabf@skynet.be>
Acked-by: Li Zefan <lizefan@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>