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Date: Wed, 29 Feb 2012 07:55:00 -0800
From: Mandeep Singh Baines <msb@...omium.org>
To: Frederic Weisbecker <fweisbec@...il.com>
Cc: Li Zefan <lizf@...fujitsu.com>, Tejun Heo <tj@...nel.org>,
LKML <linux-kernel@...r.kernel.org>,
Oleg Nesterov <oleg@...hat.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Mandeep Singh Baines <msb@...omium.org>
Subject: Re: [RFC][PATCH v2] cgroups: Run subsystem fork callback from
cgroup_post_fork()
Frederic Weisbecker (fweisbec@...il.com) wrote:
> When a user freezes a cgroup, the freezer sets the subsystem state
> to CGROUP_FREEZING and then iterates over the tasks in the cgroup links.
>
> But there is a possible race here, although unlikely, if a task
> forks and the parent is preempted between write_unlock(tasklist_lock)
> and cgroup_post_fork(). If we freeze the cgroup while the parent
So what if you moved cgroup_post_forks() a few lines up to be
inside the tasklist_lock?
I agree with you on the race and believe your solution is correct.
> is sleeping and the parent wakes up thereafter, its child will
> be missing from the set of tasks to freeze because:
>
> - The child was not yet linked to its css_set->tasks, as is done
> from cgroup_post_fork(). cgroup_iter_start() has thus missed it.
>
> - The cgroup freezer's fork callback can handle that child but
> cgroup_fork_callbacks() has been called already.
>
> One way to fix this is to call the fork callbacks after we link
> the task to the css set. The cgroup freezer is the only user of
> this callback anyway.
>
> v2: Keep the call to cgroup_exit to put the css_set on fork error.
>
> Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
> Cc: Li Zefan <lizf@...fujitsu.com>
> Cc: Tejun Heo <tj@...nel.org>
> Cc: Oleg Nesterov <oleg@...hat.com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Mandeep Singh Baines <msb@...omium.org>
> ---
>
> Not sure this is the right solution, especially as I still need
> a cancellable fork callback for my task counter and for this I
> need the fork callbacks to be called before the task is added
> on the tasklist. But anyway at least that reports this race.
>
I'm new to the task counter stuff. Would you mind providing a
reference.
Regards,
Mandeep
> kernel/cgroup.c | 39 ++++++++++++++-------------------------
> kernel/fork.c | 9 +--------
> 2 files changed, 15 insertions(+), 33 deletions(-)
>
> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
> index c6877fe..de21e52 100644
> --- a/kernel/cgroup.c
> +++ b/kernel/cgroup.c
> @@ -4496,31 +4496,6 @@ void cgroup_fork(struct task_struct *child)
> }
>
> /**
> - * 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;
> - /*
> - * forkexit callbacks are only supported for builtin
> - * subsystems, and the builtin section of the subsys array is
> - * immutable, so we don't need to lock the subsys array here.
> - */
> - for (i = 0; i < CGROUP_BUILTIN_SUBSYS_COUNT; i++) {
> - struct cgroup_subsys *ss = subsys[i];
> - 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
> *
> @@ -4559,6 +4534,20 @@ void cgroup_post_fork(struct task_struct *child)
> }
> write_unlock(&css_set_lock);
> }
> +
> + if (need_forkexit_callback) {
> + int i;
> + /*
> + * forkexit callbacks are only supported for builtin
> + * subsystems, and the builtin section of the subsys array is
> + * immutable, so we don't need to lock the subsys array here.
> + */
> + for (i = 0; i < CGROUP_BUILTIN_SUBSYS_COUNT; i++) {
> + struct cgroup_subsys *ss = subsys[i];
> + if (ss->fork)
> + ss->fork(child);
> + }
> + }
> }
> /**
> * cgroup_exit - detach cgroup from exiting task
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 051f090..551cfe0 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -1053,7 +1053,6 @@ 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);
> @@ -1305,12 +1304,6 @@ static struct task_struct *copy_process(unsigned long clone_flags,
> p->group_leader = p;
> INIT_LIST_HEAD(&p->thread_group);
>
> - /* 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);
>
> @@ -1413,7 +1406,7 @@ 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:
> --
> 1.7.5.4
>
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