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Message-ID: <CALdu-PBvOFjYC-KUF=ehc-2fTYZArQ2YO0Hd0k9nq2Chcx9cpQ@mail.gmail.com>
Date: Fri, 19 Aug 2011 08:40:52 -0700
From: Paul Menage <menage@...gle.com>
To: Tejun Heo <tj@...nel.org>
Cc: rjw@...k.pl, linux-kernel@...r.kernel.org, arnd@...db.de,
oleg@...hat.com, Li Zefan <lizf@...fujitsu.com>
Subject: Re: [PATCH 13/16] cgroup_freezer: prepare for removal of TIF_FREEZE
On Fri, Aug 19, 2011 at 7:16 AM, Tejun Heo <tj@...nel.org> wrote:
> TIF_FREEZE will be removed soon and freezing() will directly test
> whether any freezing condition is in effect. Make the following
> changes in preparation.
>
> * Rename cgroup_freezing_or_frozen() to cgroup_freezing() and make it
> return bool.
>
> * Make cgroup_freezing() access task_freezer() under rcu read lock
> instead of task_lock(). This makes the state dereferencing racy
> against task moving to another cgroup; however, it was already racy
> without this change as ->state dereference wasn't synchronized.
> This will be later dealt with using attach hooks.
>
> * freezer->state is now set before trying to push tasks into the
> target state.
>
> Signed-off-by: Tejun Heo <tj@...nel.org>
> Cc: Paul Menage <menage@...gle.com>
Acked-by: Paul Menage <paul@...lmenage.org>
> Cc: Li Zefan <lizf@...fujitsu.com>
> ---
> include/linux/freezer.h | 6 +++---
> kernel/cgroup_freezer.c | 36 ++++++++++++------------------------
> kernel/power/process.c | 2 +-
> 3 files changed, 16 insertions(+), 28 deletions(-)
>
> diff --git a/include/linux/freezer.h b/include/linux/freezer.h
> index 13559a8..bcc0637 100644
> --- a/include/linux/freezer.h
> +++ b/include/linux/freezer.h
> @@ -63,11 +63,11 @@ extern bool freeze_task(struct task_struct *p, bool sig_only);
> extern bool __set_freezable(bool with_signal);
>
> #ifdef CONFIG_CGROUP_FREEZER
> -extern int cgroup_freezing_or_frozen(struct task_struct *task);
> +extern bool cgroup_freezing(struct task_struct *task);
> #else /* !CONFIG_CGROUP_FREEZER */
> -static inline int cgroup_freezing_or_frozen(struct task_struct *task)
> +static inline bool cgroup_freezing(struct task_struct *task)
> {
> - return 0;
> + return false;
> }
> #endif /* !CONFIG_CGROUP_FREEZER */
>
> diff --git a/kernel/cgroup_freezer.c b/kernel/cgroup_freezer.c
> index e7fa0ec..0478c35 100644
> --- a/kernel/cgroup_freezer.c
> +++ b/kernel/cgroup_freezer.c
> @@ -48,19 +48,17 @@ static inline struct freezer *task_freezer(struct task_struct *task)
> struct freezer, css);
> }
>
> -static inline int __cgroup_freezing_or_frozen(struct task_struct *task)
> +bool cgroup_freezing(struct task_struct *task)
> {
> - enum freezer_state state = task_freezer(task)->state;
> - return (state == CGROUP_FREEZING) || (state == CGROUP_FROZEN);
> -}
> + enum freezer_state state;
> + bool ret;
>
> -int cgroup_freezing_or_frozen(struct task_struct *task)
> -{
> - int result;
> - task_lock(task);
> - result = __cgroup_freezing_or_frozen(task);
> - task_unlock(task);
> - return result;
> + rcu_read_lock();
> + state = task_freezer(task)->state;
> + ret = state == CGROUP_FREEZING || state == CGROUP_FROZEN;
> + rcu_read_unlock();
> +
> + return ret;
> }
>
> /*
> @@ -102,9 +100,6 @@ struct cgroup_subsys freezer_subsys;
> * freezer_can_attach():
> * cgroup_mutex (held by caller of can_attach)
> *
> - * cgroup_freezing_or_frozen():
> - * task->alloc_lock (to get task's cgroup)
> - *
> * freezer_fork() (preserving fork() performance means can't take cgroup_mutex):
> * freezer->lock
> * sighand->siglock (if the cgroup is freezing)
> @@ -177,13 +172,7 @@ static int freezer_can_attach(struct cgroup_subsys *ss,
>
> static int freezer_can_attach_task(struct cgroup *cgrp, struct task_struct *tsk)
> {
> - rcu_read_lock();
> - if (__cgroup_freezing_or_frozen(tsk)) {
> - rcu_read_unlock();
> - return -EBUSY;
> - }
> - rcu_read_unlock();
> - return 0;
> + return cgroup_freezing(tsk) ? -EBUSY : 0;
> }
>
> static void freezer_fork(struct cgroup_subsys *ss, struct task_struct *task)
> @@ -279,7 +268,6 @@ static int try_to_freeze_cgroup(struct cgroup *cgroup, struct freezer *freezer)
> struct task_struct *task;
> unsigned int num_cant_freeze_now = 0;
>
> - freezer->state = CGROUP_FREEZING;
> cgroup_iter_start(cgroup, &it);
> while ((task = cgroup_iter_next(cgroup, &it))) {
> if (!freeze_task(task, true))
> @@ -303,8 +291,6 @@ static void unfreeze_cgroup(struct cgroup *cgroup, struct freezer *freezer)
> while ((task = cgroup_iter_next(cgroup, &it)))
> __thaw_task(task);
> cgroup_iter_end(cgroup, &it);
> -
> - freezer->state = CGROUP_THAWED;
> }
>
> static int freezer_change_state(struct cgroup *cgroup,
> @@ -321,6 +307,8 @@ static int freezer_change_state(struct cgroup *cgroup,
> if (goal_state == freezer->state)
> goto out;
>
> + freezer->state = goal_state;
> +
> switch (goal_state) {
> case CGROUP_THAWED:
> unfreeze_cgroup(cgroup, freezer);
> diff --git a/kernel/power/process.c b/kernel/power/process.c
> index f8012f2..1bbc363 100644
> --- a/kernel/power/process.c
> +++ b/kernel/power/process.c
> @@ -154,7 +154,7 @@ void thaw_processes(void)
>
> read_lock(&tasklist_lock);
> do_each_thread(g, p) {
> - if (cgroup_freezing_or_frozen(p))
> + if (cgroup_freezing(p))
> continue;
>
> __thaw_task(p);
> --
> 1.7.6
>
> --
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