cgroup_freezer: allow moving tasks in and out of a frozen cgroup

cgroup_freezer is one of the few users of cgroup_subsys->can_attach()
and uses it to prevent tasks from being migrated into or out of a
frozen cgroup.  This makes cgroup_freezer cumbersome to use especially
when co-mounted with other controllers.

->can_attach() is problematic in general as it can make co-mounting
multiple cgroups difficult - migrating tasks may fail for reasons
completely irrelevant for other controllers.  freezer_can_attach() in
particular is more problematic because it messes with cgroup internal
locking to ensure that the state verification performed at
freezer_can_attach() stays valid until migration is complete.

This patch replaces freezer_can_attach() with freezer_attach() so that
tasks are always allowed to migrate - they are nudged into the
conforming state from freezer_attach().  This means that there can be
tasks which are being migrated which don't conform to the current
cgroup_freezer state until freezer_attach() is complete.  Under the
current locking scheme, the only such place is freezer_fork() which is
updated to handle such window.

While this patch doesn't remove the use of internal cgroup locking
from freezer_read/write() paths, it removes the requirement to keep
the freezer state constant while migrating and enables such change.

Note that this creates a userland visible behavior change - FROZEN
cgroup can no longer be used to lock migrations in and out of the
cgroup.  This behavior change is intended.  I don't think the feature
is necessary - userland should coordinate accesses to cgroup fs anyway
- and even if the feature is needed cgroup_freezer is the completely
wrong place to implement it.

Signed-off-by: Tejun Heo <tj@kernel.org>
LKML-Reference: <1350426526-14254-1-git-send-email-tj@kernel.org>
Cc: Matt Helsley <matthltc@linux.vnet.ibm.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Rafael J. Wysocki <rjw@sisk.pl>
Cc: Li Zefan <lizefan@huawei.com>

1 files changed, 31 insertions, 20 deletions

diff --git a/kernel/cgroup_freezer.c b/kernel/cgroup_freezer.c
index 557f3678c4e4..0b0e10545ef0 100644
--- a/kernel/cgroup_freezer.c
+++ b/kernel/cgroup_freezer.c
@@ -152,27 +152,38 @@ static void freezer_destroy(struct cgroup *cgroup)
 
 /*
  * The call to cgroup_lock() in the freezer.state write method prevents
- * a write to that file racing against an attach, and hence the
- * can_attach() result will remain valid until the attach completes.
+ * a write to that file racing against an attach, and hence we don't need
+ * to worry about racing against migration.
  */
-static int freezer_can_attach(struct cgroup *new_cgroup,
-                              struct cgroup_taskset *tset)
+static void freezer_attach(struct cgroup *new_cgrp, struct cgroup_taskset *tset)
 {
-        struct freezer *freezer;
+        struct freezer *freezer = cgroup_freezer(new_cgrp);
         struct task_struct *task;
 
+        spin_lock_irq(&freezer->lock);
+
         /*
-         * Anything frozen can't move or be moved to/from.
+         * Make the new tasks conform to the current state of @new_cgrp.
+         * For simplicity, when migrating any task to a FROZEN cgroup, we
+         * revert it to FREEZING and let update_if_frozen() determine the
+         * correct state later.
+         *
+         * Tasks in @tset are on @new_cgrp but may not conform to its
+         * current state before executing the following - !frozen tasks may
+         * be visible in a FROZEN cgroup and frozen tasks in a THAWED one.
+         * This means that, to determine whether to freeze, one should test
+         * whether the state equals THAWED.
          */
-        cgroup_taskset_for_each(task, new_cgroup, tset)
-                if (cgroup_freezing(task))
-                        return -EBUSY;
-
-        freezer = cgroup_freezer(new_cgroup);
-        if (freezer->state != CGROUP_THAWED)
-                return -EBUSY;
+        cgroup_taskset_for_each(task, new_cgrp, tset) {
+                if (freezer->state == CGROUP_THAWED) {
+                        __thaw_task(task);
+                } else {
+                        freeze_task(task);
+                        freezer->state = CGROUP_FREEZING;
+                }
+        }
 
-        return 0;
+        spin_unlock_irq(&freezer->lock);
 }
 
 static void freezer_fork(struct task_struct *task)
@@ -190,12 +201,12 @@ static void freezer_fork(struct task_struct *task)
                 goto out;
 
         spin_lock_irq(&freezer->lock);
-        BUG_ON(freezer->state == CGROUP_FROZEN);
-
-        /* Locking avoids race with FREEZING -> THAWED transitions. */
-        if (freezer->state == CGROUP_FREEZING)
+        /*
+         * @task might have been just migrated into a FROZEN cgroup.  Test
+         * equality with THAWED.  Read the comment in freezer_attach().
+         */
+        if (freezer->state != CGROUP_THAWED)
                 freeze_task(task);
-
         spin_unlock_irq(&freezer->lock);
 out:
         rcu_read_unlock();
@@ -352,7 +363,7 @@ struct cgroup_subsys freezer_subsys = {
         .create         = freezer_create,
         .destroy        = freezer_destroy,
         .subsys_id      = freezer_subsys_id,
-        .can_attach     = freezer_can_attach,
+        .attach         = freezer_attach,
         .fork           = freezer_fork,
         .base_cftypes   = files,