cgroup: cgroup_subsys->fork() should be called after the task is added to css_set

cgroup core has a bug which violates a basic rule about event
notifications - when a new entity needs to be added, you add that to
the notification list first and then make the new entity conform to
the current state.  If done in the reverse order, an event happening
inbetween will be lost.

cgroup_subsys->fork() is invoked way before the new task is added to
the css_set.  Currently, cgroup_freezer is the only user of ->fork()
and uses it to make new tasks conform to the current state of the
freezer.  If FROZEN state is requested while fork is in progress
between cgroup_fork_callbacks() and cgroup_post_fork(), the child
could escape freezing - the cgroup isn't frozen when ->fork() is
called and the freezer couldn't see the new task on the css_set.

This patch moves cgroup_subsys->fork() invocation to
cgroup_post_fork() after the new task is added to the css_set.
cgroup_fork_callbacks() is removed.

Because now a task may be migrated during cgroup_subsys->fork(),
freezer_fork() is updated so that it adheres to the usual RCU locking
and the rather pointless comment on why locking can be different there
is removed (if it doesn't make anything simpler, why even bother?).

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Rafael J. Wysocki <rjw@sisk.pl>
Cc: stable@vger.kernel.org

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index f8a030ced0c7f39297085eb346a989aff5b5d612..4cd1d0fd2542c147f67413dbaec3d37e2f7f558a 100644 (file)
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -34,7 +34,6 @@ extern int cgroup_lock_is_held(void);
 extern bool cgroup_lock_live_group(struct cgroup *cgrp);
 extern void cgroup_unlock(void);
 extern void cgroup_fork(struct task_struct *p);
-extern void cgroup_fork_callbacks(struct task_struct *p);
 extern void cgroup_post_fork(struct task_struct *p);
 extern void cgroup_exit(struct task_struct *p, int run_callbacks);
 extern int cgroupstats_build(struct cgroupstats *stats,

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 13774b3b39aac9b73e25ba34a366d5402b70df00..b7a0171067ea9e4dc240507514b6ce2ae32f3ce2 100644 (file)
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -4843,45 +4843,20 @@ void cgroup_fork(struct task_struct *child)
        INIT_LIST_HEAD(&child->cg_list);
 }
 
-/**
- * cgroup_fork_callbacks - run fork callbacks
- * @child: the new task
- *
- * Called on a new task very soon before adding it to the
- * tasklist. No need to take any locks since no-one can
- * be operating on this task.
- */
-void cgroup_fork_callbacks(struct task_struct *child)
-{
-       if (need_forkexit_callback) {
-               int i;
-               for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
-                       struct cgroup_subsys *ss = subsys[i];
-
-                       /*
-                        * forkexit callbacks are only supported for
-                        * builtin subsystems.
-                        */
-                       if (!ss || ss->module)
-                               continue;
-
-                       if (ss->fork)
-                               ss->fork(child);
-               }
-       }
-}
-
 /**
  * cgroup_post_fork - called on a new task after adding it to the task list
  * @child: the task in question
  *
- * Adds the task to the list running through its css_set if necessary.
- * Has to be after the task is visible on the task list in case we race
- * with the first call to cgroup_iter_start() - to guarantee that the
- * new task ends up on its list.
+ * Adds the task to the list running through its css_set if necessary and
+ * call the subsystem fork() callbacks.  Has to be after the task is
+ * visible on the task list in case we race with the first call to
+ * cgroup_iter_start() - to guarantee that the new task ends up on its
+ * list.
  */
 void cgroup_post_fork(struct task_struct *child)
 {
+       int i;
+
        /*
         * use_task_css_set_links is set to 1 before we walk the tasklist
         * under the tasklist_lock and we read it here after we added the child
@@ -4910,7 +4885,30 @@ void cgroup_post_fork(struct task_struct *child)
                }
                write_unlock(&css_set_lock);
        }
+
+       /*
+        * Call ss->fork().  This must happen after @child is linked on
+        * css_set; otherwise, @child might change state between ->fork()
+        * and addition to css_set.
+        */
+       if (need_forkexit_callback) {
+               for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
+                       struct cgroup_subsys *ss = subsys[i];
+
+                       /*
+                        * fork/exit callbacks are supported only for
+                        * builtin subsystems and we don't need further
+                        * synchronization as they never go away.
+                        */
+                       if (!ss || ss->module)
+                               continue;
+
+                       if (ss->fork)
+                               ss->fork(child);
+               }
+       }
 }
+
 /**
  * cgroup_exit - detach cgroup from exiting task
  * @tsk: pointer to task_struct of exiting process

diff --git a/kernel/cgroup_freezer.c b/kernel/cgroup_freezer.c
index b1724ce98981d25e76980193b7e3984c9f7717cd..12bfedb598c2d5367e9b0e68e8d6cec8bf85e7a7 100644 (file)
--- a/kernel/cgroup_freezer.c
+++ b/kernel/cgroup_freezer.c
@@ -186,23 +186,15 @@ static void freezer_fork(struct task_struct *task)
 {
        struct freezer *freezer;
 
-       /*
-        * No lock is needed, since the task isn't on tasklist yet,
-        * so it can't be moved to another cgroup, which means the
-        * freezer won't be removed and will be valid during this
-        * function call.  Nevertheless, apply RCU read-side critical
-        * section to suppress RCU lockdep false positives.
-        */
        rcu_read_lock();
        freezer = task_freezer(task);
-       rcu_read_unlock();
 
        /*
         * The root cgroup is non-freezable, so we can skip the
         * following check.
         */
        if (!freezer->css.cgroup->parent)
-               return;
+               goto out;
 
        spin_lock_irq(&freezer->lock);
        BUG_ON(freezer->state == CGROUP_FROZEN);
@@ -210,7 +202,10 @@ static void freezer_fork(struct task_struct *task)
        /* Locking avoids race with FREEZING -> THAWED transitions. */
        if (freezer->state == CGROUP_FREEZING)
                freeze_task(task);
+
        spin_unlock_irq(&freezer->lock);
+out:
+       rcu_read_unlock();
 }
 
 /*

diff --git a/kernel/fork.c b/kernel/fork.c
index 8b20ab7d3aa2951eff91a4e09e0af23a90992747..acc4cb62f32fd70318b61a38cc34ac00da6911b0 100644 (file)
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1135,7 +1135,6 @@ static struct task_struct *copy_process(unsigned long clone_flags,
 {
        int retval;
        struct task_struct *p;
-       int cgroup_callbacks_done = 0;
 
        if ((clone_flags & (CLONE_NEWNS|CLONE_FS)) == (CLONE_NEWNS|CLONE_FS))
                return ERR_PTR(-EINVAL);
@@ -1393,12 +1392,6 @@ static struct task_struct *copy_process(unsigned long clone_flags,
        INIT_LIST_HEAD(&p->thread_group);
        p->task_works = NULL;
 
-       /* Now that the task is set up, run cgroup callbacks if
-        * necessary. We need to run them before the task is visible
-        * on the tasklist. */
-       cgroup_fork_callbacks(p);
-       cgroup_callbacks_done = 1;
-
        /* Need tasklist lock for parent etc handling! */
        write_lock_irq(&tasklist_lock);
 
@@ -1503,7 +1496,7 @@ bad_fork_cleanup_cgroup:
 #endif
        if (clone_flags & CLONE_THREAD)
                threadgroup_change_end(current);
-       cgroup_exit(p, cgroup_callbacks_done);
+       cgroup_exit(p, 0);
        delayacct_tsk_free(p);
        module_put(task_thread_info(p)->exec_domain->module);
 bad_fork_cleanup_count: