linux-kernel - Re: [PATCH 2/3] clone3: allow spawning processes into cgroups

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Date:   Fri, 20 Dec 2019 21:36:50 +0100
From:   Oleg Nesterov <oleg@...hat.com>
To:     Christian Brauner <christian.brauner@...ntu.com>
Cc:     linux-api@...r.kernel.org, linux-kernel@...r.kernel.org,
        Tejun Heo <tj@...nel.org>, Ingo Molnar <mingo@...hat.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Li Zefan <lizefan@...wei.com>,
        Peter Zijlstra <peterz@...radead.org>, cgroups@...r.kernel.org
Subject: Re: [PATCH 2/3] clone3: allow spawning processes into cgroups

On 12/18, Christian Brauner wrote:
>
> This adds support for creating a process in a different cgroup than its
> parent.

Cough... I will not comment the intent ;) I can't review the cgroup patches
anyway.

However,

> +int cgroup_lock_fork(struct kernel_clone_args *kargs)
> +	__acquires(&cgroup_mutex)
> +{
> +	struct cgroup *cgrp;
> +
> +	if (!(kargs->flags & CLONE_INTO_CGROUP))
> +		return 0;
> +
> +	cgrp = kargs->cgrp;
> +	if (!cgrp)
> +		return 0;
> +
> +	mutex_lock(&cgroup_mutex);
> +
> +	if (!cgroup_is_dead(cgrp))
> +		return 0;
> +
> +	mutex_unlock(&cgroup_mutex);
> +	return -ENODEV;

...

> @@ -2172,7 +2172,7 @@ static __latent_entropy struct task_struct *copy_process(
>  	 * between here and cgroup_post_fork() if an organisation operation is in
>  	 * progress.
>  	 */
> -	retval = cgroup_can_fork(p);
> +	retval = cgroup_can_fork(current, p, args);
>  	if (retval)
>  		goto bad_fork_cgroup_threadgroup_change_end;
>  
> @@ -2226,6 +2226,10 @@ static __latent_entropy struct task_struct *copy_process(
>  		goto bad_fork_cancel_cgroup;
>  	}
>  
> +	retval = cgroup_lock_fork(args);

mutex_lock() under spin_lock() ??


just in case, note that mutex_lock(&cgroup_mutex) is not safe even under
cgroup_threadgroup_change_begin(), this can deadlock.

Oleg.