cgroup_freezer: replace freezer->lock with freezer_mutex

After 96d365e0b86e ("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: 96d365e0b86e ("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>

1 files changed, 46 insertions, 66 deletions

diff --git a/kernel/cgroup_freezer.c b/kernel/cgroup_freezer.c
index 2bc4a2256444..12ead0b766ee 100644
--- a/kernel/cgroup_freezer.c
+++ b/kernel/cgroup_freezer.c
@@ -21,6 +21,7 @@
 #include <linux/uaccess.h>
 #include <linux/freezer.h>
 #include <linux/seq_file.h>
+#include <linux/mutex.h>
 
 /*
  * A cgroup is freezing if any FREEZING flags are set.  FREEZING_SELF is
@@ -42,9 +43,10 @@ enum freezer_state_flags {
 struct freezer {
         struct cgroup_subsys_state      css;
         unsigned int                    state;
-        spinlock_t                      lock;
 };
 
+static DEFINE_MUTEX(freezer_mutex);
+
 static inline struct freezer *css_freezer(struct cgroup_subsys_state *css)
 {
         return css ? container_of(css, struct freezer, css) : NULL;
@@ -93,7 +95,6 @@ freezer_css_alloc(struct cgroup_subsys_state *parent_css)
         if (!freezer)
                 return ERR_PTR(-ENOMEM);
 
-        spin_lock_init(&freezer->lock);
         return &freezer->css;
 }
 
@@ -110,14 +111,7 @@ static int freezer_css_online(struct cgroup_subsys_state *css)
         struct freezer *freezer = css_freezer(css);
         struct freezer *parent = parent_freezer(freezer);
 
-        /*
-         * The following double locking and freezing state inheritance
-         * guarantee that @cgroup can never escape ancestors' freezing
-         * states.  See css_for_each_descendant_pre() for details.
-         */
-        if (parent)
-                spin_lock_irq(&parent->lock);
-        spin_lock_nested(&freezer->lock, SINGLE_DEPTH_NESTING);
+        mutex_lock(&freezer_mutex);
 
         freezer->state |= CGROUP_FREEZER_ONLINE;
 
@@ -126,10 +120,7 @@ static int freezer_css_online(struct cgroup_subsys_state *css)
                 atomic_inc(&system_freezing_cnt);
         }
 
-        spin_unlock(&freezer->lock);
-        if (parent)
-                spin_unlock_irq(&parent->lock);
-
+        mutex_unlock(&freezer_mutex);
         return 0;
 }
 
@@ -144,14 +135,14 @@ static void freezer_css_offline(struct cgroup_subsys_state *css)
 {
         struct freezer *freezer = css_freezer(css);
 
-        spin_lock_irq(&freezer->lock);
+        mutex_lock(&freezer_mutex);
 
         if (freezer->state & CGROUP_FREEZING)
                 atomic_dec(&system_freezing_cnt);
 
         freezer->state = 0;
 
-        spin_unlock_irq(&freezer->lock);
+        mutex_unlock(&freezer_mutex);
 }
 
 static void freezer_css_free(struct cgroup_subsys_state *css)
@@ -175,7 +166,7 @@ static void freezer_attach(struct cgroup_subsys_state *new_css,
         struct task_struct *task;
         bool clear_frozen = false;
 
-        spin_lock_irq(&freezer->lock);
+        mutex_lock(&freezer_mutex);
 
         /*
          * Make the new tasks conform to the current state of @new_css.
@@ -197,21 +188,13 @@ static void freezer_attach(struct cgroup_subsys_state *new_css,
                 }
         }
 
-        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.
-         */
+        /* propagate FROZEN clearing upwards */
         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);
         }
+
+        mutex_unlock(&freezer_mutex);
 }
 
 /**
@@ -228,9 +211,6 @@ static void freezer_fork(struct task_struct *task)
 {
         struct freezer *freezer;
 
-        rcu_read_lock();
-        freezer = task_freezer(task);
-
         /*
          * The root cgroup is non-freezable, so we can skip locking the
          * freezer.  This is safe regardless of race with task migration.
@@ -238,24 +218,18 @@ static void freezer_fork(struct task_struct *task)
          * to do.  If we lost and root is the new cgroup, noop is still the
          * right thing to do.
          */
-        if (!parent_freezer(freezer))
-                goto out;
+        if (task_css_is_root(task, freezer_cgrp_id))
+                return;
 
-        /*
-         * Grab @freezer->lock and freeze @task after verifying @task still
-         * belongs to @freezer and it's freezing.  The former is for the
-         * case where we have raced against task migration and lost and
-         * @task is already in a different cgroup which may not be frozen.
-         * This isn't strictly necessary as freeze_task() is allowed to be
-         * called spuriously but let's do it anyway for, if nothing else,
-         * documentation.
-         */
-        spin_lock_irq(&freezer->lock);
-        if (freezer == task_freezer(task) && (freezer->state & CGROUP_FREEZING))
+        mutex_lock(&freezer_mutex);
+        rcu_read_lock();
+
+        freezer = task_freezer(task);
+        if (freezer->state & CGROUP_FREEZING)
                 freeze_task(task);
-        spin_unlock_irq(&freezer->lock);
-out:
+
         rcu_read_unlock();
+        mutex_unlock(&freezer_mutex);
 }
 
 /**
@@ -281,22 +255,22 @@ static void update_if_frozen(struct cgroup_subsys_state *css)
         struct css_task_iter it;
         struct task_struct *task;
 
-        WARN_ON_ONCE(!rcu_read_lock_held());
-
-        spin_lock_irq(&freezer->lock);
+        lockdep_assert_held(&freezer_mutex);
 
         if (!(freezer->state & CGROUP_FREEZING) ||
             (freezer->state & CGROUP_FROZEN))
-                goto out_unlock;
+                return;
 
         /* are all (live) children frozen? */
+        rcu_read_lock();
         css_for_each_child(pos, css) {
                 struct freezer *child = css_freezer(pos);
 
                 if ((child->state & CGROUP_FREEZER_ONLINE) &&
                     !(child->state & CGROUP_FROZEN))
-                        goto out_unlock;
+                        return;
         }
+        rcu_read_unlock();
 
         /* are all tasks frozen? */
         css_task_iter_start(css, &it);
@@ -317,21 +291,29 @@ static void update_if_frozen(struct cgroup_subsys_state *css)
         freezer->state |= CGROUP_FROZEN;
 out_iter_end:
         css_task_iter_end(&it);
-out_unlock:
-        spin_unlock_irq(&freezer->lock);
 }
 
 static int freezer_read(struct seq_file *m, void *v)
 {
         struct cgroup_subsys_state *css = seq_css(m), *pos;
 
+        mutex_lock(&freezer_mutex);
         rcu_read_lock();
 
         /* update states bottom-up */
-        css_for_each_descendant_post(pos, css)
+        css_for_each_descendant_post(pos, css) {
+                if (!css_tryget(pos))
+                        continue;
+                rcu_read_unlock();
+
                 update_if_frozen(pos);
 
+                rcu_read_lock();
+                css_put(pos);
+        }
+
         rcu_read_unlock();
+        mutex_unlock(&freezer_mutex);
 
         seq_puts(m, freezer_state_strs(css_freezer(css)->state));
         seq_putc(m, '\n');
@@ -373,7 +355,7 @@ static void freezer_apply_state(struct freezer *freezer, bool freeze,
                                 unsigned int state)
 {
         /* also synchronizes against task migration, see freezer_attach() */
-        lockdep_assert_held(&freezer->lock);
+        lockdep_assert_held(&freezer_mutex);
 
         if (!(freezer->state & CGROUP_FREEZER_ONLINE))
                 return;
@@ -414,31 +396,29 @@ static void freezer_change_state(struct freezer *freezer, bool freeze)
          * descendant will try to inherit its parent's FREEZING state as
          * CGROUP_FREEZING_PARENT.
          */
+        mutex_lock(&freezer_mutex);
         rcu_read_lock();
         css_for_each_descendant_pre(pos, &freezer->css) {
                 struct freezer *pos_f = css_freezer(pos);
                 struct freezer *parent = parent_freezer(pos_f);
 
-                spin_lock_irq(&pos_f->lock);
+                if (!css_tryget(pos))
+                        continue;
+                rcu_read_unlock();
 
-                if (pos_f == freezer) {
+                if (pos_f == freezer)
                         freezer_apply_state(pos_f, freeze,
                                             CGROUP_FREEZING_SELF);
-                } else {
-                        /*
-                         * 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().
-                         */
+                else
                         freezer_apply_state(pos_f,
                                             parent->state & CGROUP_FREEZING,
                                             CGROUP_FREEZING_PARENT);
-                }
 
-                spin_unlock_irq(&pos_f->lock);
+                rcu_read_lock();
+                css_put(pos);
         }
         rcu_read_unlock();
+        mutex_unlock(&freezer_mutex);
 }
 
 static int freezer_write(struct cgroup_subsys_state *css, struct cftype *cft,