cgroup_freezer: implement proper hierarchy support

Up until now, cgroup_freezer didn't implement hierarchy properly.
cgroups could be arranged in hierarchy but it didn't make any
difference in how each cgroup_freezer behaved.  They all operated
separately.

This patch implements proper hierarchy support.  If a cgroup is
frozen, all its descendants are frozen.  A cgroup is thawed iff it and
all its ancestors are THAWED.  freezer.self_freezing shows the current
freezing state for the cgroup itself.  freezer.parent_freezing shows
whether the cgroup is freezing because any of its ancestors is
freezing.

freezer_post_create() locks the parent and new cgroup and inherits the
parent's state and freezer_change_state() applies new state top-down
using cgroup_for_each_descendant_pre() which guarantees that no child
can escape its parent's state.  update_if_frozen() uses
cgroup_for_each_descendant_post() to propagate frozen states
bottom-up.

Synchronization could be coarser and easier by using a single mutex to
protect all hierarchy operations.  Finer grained approach was used
because it wasn't too difficult for cgroup_freezer and I think it's
beneficial to have an example implementation and cgroup_freezer is
rather simple and can serve a good one.

As this makes cgroup_freezer properly hierarchical,
freezer_subsys.broken_hierarchy marking is removed.

Note that this patch changes userland visible behavior - freezing a
cgroup now freezes all its descendants too.  This behavior change is
intended and has been warned via .broken_hierarchy.

v2: Michal spotted a bug in freezer_change_state() - descendants were
    inheriting from the wrong ancestor.  Fixed.

v3: Documentation/cgroups/freezer-subsystem.txt updated.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Michal Hocko <mhocko@suse.cz>

1 files changed, 123 insertions, 38 deletions

diff --git a/kernel/cgroup_freezer.c b/kernel/cgroup_freezer.c
index 4f12d317c4c3..670a4af7dc94 100644
--- a/kernel/cgroup_freezer.c
+++ b/kernel/cgroup_freezer.c
@@ -22,6 +22,13 @@
 #include <linux/freezer.h>
 #include <linux/seq_file.h>
 
+/*
+ * A cgroup is freezing if any FREEZING flags are set.  FREEZING_SELF is
+ * set if "FROZEN" is written to freezer.state cgroupfs file, and cleared
+ * for "THAWED".  FREEZING_PARENT is set if the parent freezer is FREEZING
+ * for whatever reason.  IOW, a cgroup has FREEZING_PARENT set if one of
+ * its ancestors has FREEZING_SELF set.
+ */
 enum freezer_state_flags {
         CGROUP_FREEZER_ONLINE   = (1 << 0), /* freezer is fully online */
         CGROUP_FREEZING_SELF    = (1 << 1), /* this freezer is freezing */
@@ -50,6 +57,15 @@ static inline struct freezer *task_freezer(struct task_struct *task)
                             struct freezer, css);
 }
 
+static struct freezer *parent_freezer(struct freezer *freezer)
+{
+        struct cgroup *pcg = freezer->css.cgroup->parent;
+
+        if (pcg)
+                return cgroup_freezer(pcg);
+        return NULL;
+}
+
 bool cgroup_freezing(struct task_struct *task)
 {
         bool ret;
@@ -74,17 +90,6 @@ static const char *freezer_state_strs(unsigned int state)
         return "THAWED";
 };
 
-/*
- * State diagram
- * Transitions are caused by userspace writes to the freezer.state file.
- * The values in parenthesis are state labels. The rest are edge labels.
- *
- * (THAWED) --FROZEN--> (FREEZING) --FROZEN--> (FROZEN)
- *    ^ ^                    |                     |
- *    | \_______THAWED_______/                     |
- *    \__________________________THAWED____________/
- */
-
 struct cgroup_subsys freezer_subsys;
 
 static struct cgroup_subsys_state *freezer_create(struct cgroup *cgroup)
@@ -103,15 +108,34 @@ static struct cgroup_subsys_state *freezer_create(struct cgroup *cgroup)
  * freezer_post_create - commit creation of a freezer cgroup
  * @cgroup: cgroup being created
  *
- * We're committing to creation of @cgroup.  Mark it online.
+ * We're committing to creation of @cgroup.  Mark it online and inherit
+ * parent's freezing state while holding both parent's and our
+ * freezer->lock.
  */
 static void freezer_post_create(struct cgroup *cgroup)
 {
         struct freezer *freezer = cgroup_freezer(cgroup);
+        struct freezer *parent = parent_freezer(freezer);
+
+        /*
+         * The following double locking and freezing state inheritance
+         * guarantee that @cgroup can never escape ancestors' freezing
+         * states.  See cgroup_for_each_descendant_pre() for details.
+         */
+        if (parent)
+                spin_lock_irq(&parent->lock);
+        spin_lock_nested(&freezer->lock, SINGLE_DEPTH_NESTING);
 
-        spin_lock_irq(&freezer->lock);
         freezer->state |= CGROUP_FREEZER_ONLINE;
-        spin_unlock_irq(&freezer->lock);
+
+        if (parent && (parent->state & CGROUP_FREEZING)) {
+                freezer->state |= CGROUP_FREEZING_PARENT | CGROUP_FROZEN;
+                atomic_inc(&system_freezing_cnt);
+        }
+
+        spin_unlock(&freezer->lock);
+        if (parent)
+                spin_unlock_irq(&parent->lock);
 }
 
 /**
@@ -153,6 +177,7 @@ static void freezer_attach(struct cgroup *new_cgrp, struct cgroup_taskset *tset)
 {
         struct freezer *freezer = cgroup_freezer(new_cgrp);
         struct task_struct *task;
+        bool clear_frozen = false;
 
         spin_lock_irq(&freezer->lock);
 
@@ -172,10 +197,25 @@ static void freezer_attach(struct cgroup *new_cgrp, struct cgroup_taskset *tset)
                 } else {
                         freeze_task(task);
                         freezer->state &= ~CGROUP_FROZEN;
+                        clear_frozen = true;
                 }
         }
 
         spin_unlock_irq(&freezer->lock);
+
+        /*
+         * Propagate FROZEN clearing upwards.  We may race with
+         * update_if_frozen(), but as long as both work bottom-up, either
+         * update_if_frozen() sees child's FROZEN cleared or we clear the
+         * parent's FROZEN later.  No parent w/ !FROZEN children can be
+         * left FROZEN.
+         */
+        while (clear_frozen && (freezer = parent_freezer(freezer))) {
+                spin_lock_irq(&freezer->lock);
+                freezer->state &= ~CGROUP_FROZEN;
+                clear_frozen = freezer->state & CGROUP_FREEZING;
+                spin_unlock_irq(&freezer->lock);
+        }
 }
 
 static void freezer_fork(struct task_struct *task)
@@ -200,24 +240,47 @@ out:
         rcu_read_unlock();
 }
 
-/*
- * We change from FREEZING to FROZEN lazily if the cgroup was only
- * partially frozen when we exitted write.  Caller must hold freezer->lock.
+/**
+ * update_if_frozen - update whether a cgroup finished freezing
+ * @cgroup: cgroup of interest
+ *
+ * Once FREEZING is initiated, transition to FROZEN is lazily updated by
+ * calling this function.  If the current state is FREEZING but not FROZEN,
+ * this function checks whether all tasks of this cgroup and the descendant
+ * cgroups finished freezing and, if so, sets FROZEN.
+ *
+ * The caller is responsible for grabbing RCU read lock and calling
+ * update_if_frozen() on all descendants prior to invoking this function.
  *
  * Task states and freezer state might disagree while tasks are being
  * migrated into or out of @cgroup, so we can't verify task states against
  * @freezer state here.  See freezer_attach() for details.
  */
-static void update_if_frozen(struct freezer *freezer)
+static void update_if_frozen(struct cgroup *cgroup)
 {
-        struct cgroup *cgroup = freezer->css.cgroup;
+        struct freezer *freezer = cgroup_freezer(cgroup);
+        struct cgroup *pos;
         struct cgroup_iter it;
         struct task_struct *task;
 
+        WARN_ON_ONCE(!rcu_read_lock_held());
+
+        spin_lock_irq(&freezer->lock);
+
         if (!(freezer->state & CGROUP_FREEZING) ||
             (freezer->state & CGROUP_FROZEN))
-                return;
+                goto out_unlock;
+
+        /* are all (live) children frozen? */
+        cgroup_for_each_child(pos, cgroup) {
+                struct freezer *child = cgroup_freezer(pos);
 
+                if ((child->state & CGROUP_FREEZER_ONLINE) &&
+                    !(child->state & CGROUP_FROZEN))
+                        goto out_unlock;
+        }
+
+        /* are all tasks frozen? */
         cgroup_iter_start(cgroup, &it);
 
         while ((task = cgroup_iter_next(cgroup, &it))) {
@@ -229,27 +292,32 @@ static void update_if_frozen(struct freezer *freezer)
                          * the usual frozen condition.
                          */
                         if (!frozen(task) && !freezer_should_skip(task))
-                                goto notyet;
+                                goto out_iter_end;
                 }
         }
 
         freezer->state |= CGROUP_FROZEN;
-notyet:
+out_iter_end:
         cgroup_iter_end(cgroup, &it);
+out_unlock:
+        spin_unlock_irq(&freezer->lock);
 }
 
 static int freezer_read(struct cgroup *cgroup, struct cftype *cft,
                         struct seq_file *m)
 {
-        struct freezer *freezer = cgroup_freezer(cgroup);
-        unsigned int state;
+        struct cgroup *pos;
 
-        spin_lock_irq(&freezer->lock);
-        update_if_frozen(freezer);
-        state = freezer->state;
-        spin_unlock_irq(&freezer->lock);
+        rcu_read_lock();
 
-        seq_puts(m, freezer_state_strs(state));
+        /* update states bottom-up */
+        cgroup_for_each_descendant_post(pos, cgroup)
+                update_if_frozen(pos);
+        update_if_frozen(cgroup);
+
+        rcu_read_unlock();
+
+        seq_puts(m, freezer_state_strs(cgroup_freezer(cgroup)->state));
         seq_putc(m, '\n');
         return 0;
 }
@@ -320,14 +388,39 @@ static void freezer_apply_state(struct freezer *freezer, bool freeze,
  * @freezer: freezer of interest
  * @freeze: whether to freeze or thaw
  *
- * Freeze or thaw @cgroup according to @freeze.
+ * Freeze or thaw @freezer according to @freeze.  The operations are
+ * recursive - all descendants of @freezer will be affected.
  */
 static void freezer_change_state(struct freezer *freezer, bool freeze)
 {
+        struct cgroup *pos;
+
         /* update @freezer */
         spin_lock_irq(&freezer->lock);
         freezer_apply_state(freezer, freeze, CGROUP_FREEZING_SELF);
         spin_unlock_irq(&freezer->lock);
+
+        /*
+         * Update all its descendants in pre-order traversal.  Each
+         * descendant will try to inherit its parent's FREEZING state as
+         * CGROUP_FREEZING_PARENT.
+         */
+        rcu_read_lock();
+        cgroup_for_each_descendant_pre(pos, freezer->css.cgroup) {
+                struct freezer *pos_f = cgroup_freezer(pos);
+                struct freezer *parent = parent_freezer(pos_f);
+
+                /*
+                 * Our update to @parent->state is already visible which is
+                 * all we need.  No need to lock @parent.  For more info on
+                 * synchronization, see freezer_post_create().
+                 */
+                spin_lock_irq(&pos_f->lock);
+                freezer_apply_state(pos_f, parent->state & CGROUP_FREEZING,
+                                    CGROUP_FREEZING_PARENT);
+                spin_unlock_irq(&pos_f->lock);
+        }
+        rcu_read_unlock();
 }
 
 static int freezer_write(struct cgroup *cgroup, struct cftype *cft,
@@ -390,12 +483,4 @@ struct cgroup_subsys freezer_subsys = {
         .attach         = freezer_attach,
         .fork           = freezer_fork,
         .base_cftypes   = files,
-
-        /*
-         * freezer subsys doesn't handle hierarchy at all.  Frozen state
-         * should be inherited through the hierarchy - if a parent is
-         * frozen, all its children should be frozen.  Fix it and remove
-         * the following.
-         */
-        .broken_hierarchy = true,
 };