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cgroup_freezer: document freezer_fork() subtleties

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cgroup_subsys->fork() callback is special in that it's called outside
the usual cgroup locking and may race with on-going migration.
freezer_fork() currently doesn't consider such race condition;
however, it is still correct thanks to the fact that freeze_task() may
be called spuriously.

This is quite subtle.  Let's explain what's going on and add test to
detect racing and losing to task migration and skip freeze_task() in
such cases for documentation.

This doesn't make any behavior difference meaningful to userland.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Li Zefan <lizefan@huawei.com>
Cc: "Rafael J. Wysocki" <rjw@rjwysocki.net>
This commit is contained in:
Tejun Heo 2014-02-12 16:07:59 -05:00
parent 952aaa1254
commit a60bed296a
1 changed files with 25 additions and 3 deletions
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28
kernel/cgroup_freezer.c
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@ -214,6 +214,16 @@ static void freezer_attach(struct cgroup_subsys_state *new_css,
}
}
/**
* freezer_fork - cgroup post fork callback
* @task: a task which has just been forked
*
* @task has just been created and should conform to the current state of
* the cgroup_freezer it belongs to. This function may race against
* freezer_attach(). Losing to freezer_attach() means that we don't have
* to do anything as freezer_attach() will put @task into the appropriate
* state.
*/
static void freezer_fork(struct task_struct *task)
{
struct freezer *freezer;
@ -222,14 +232,26 @@ static void freezer_fork(struct task_struct *task)
freezer = task_freezer(task);
/*
* The root cgroup is non-freezable, so we can skip the
* following check.
* The root cgroup is non-freezable, so we can skip locking the
* freezer. This is safe regardless of race with task migration.
* If we didn't race or won, skipping is obviously the right thing
* 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;
/*
* 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->state & CGROUP_FREEZING)
if (freezer == task_freezer(task) && (freezer->state & CGROUP_FREEZING))
freeze_task(task);
spin_unlock_irq(&freezer->lock);
out: