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Date: Thu, 28 May 2009 01:19:50 +0400
From: Alexey Dobriyan <adobriyan@gmail.com>
To: Oren Laadan <orenl@cs.columbia.edu>
Cc: Andrew Morton <akpm@linux-foundation.org>,
       Linus Torvalds <torvalds@osdl.org>,
       containers@lists.linux-foundation.org, linux-kernel@vger.kernel.org,
       linux-mm@kvack.org, linux-api@vger.kernel.org,
       Serge Hallyn <serue@us.ibm.com>, Dave Hansen <dave@linux.vnet.ibm.com>,
       Ingo Molnar <mingo@elte.hu>, "H. Peter Anvin" <hpa@zytor.com>,
       Alexander Viro <viro@zeniv.linux.org.uk>,
       Pavel Emelyanov <xemul@openvz.org>
Subject: Re: [RFC v16][PATCH 19/43] c/r: external checkpoint of a task
	other than ourself
Message-ID: <20090527211950.GA7855@x200.localdomain>
References: <1243445589-32388-1-git-send-email-orenl@cs.columbia.edu> <1243445589-32388-20-git-send-email-orenl@cs.columbia.edu>
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On Wed, May 27, 2009 at 01:32:45PM -0400, Oren Laadan wrote:
> Now we can do "external" checkpoint, i.e. act on another task.

> +static int may_checkpoint_task(struct ckpt_ctx *ctx, struct task_struct *t)
> +{
> +	if (t->state == TASK_DEAD) {
> +		pr_warning("c/r: task %d is TASK_DEAD\n", task_pid_vnr(t));
> +		return -EAGAIN;
> +	}
> +
> +	if (!ptrace_may_access(t, PTRACE_MODE_READ)) {
> +		__ckpt_write_err(ctx, "access to task %d (%s) denied",
> +				 task_pid_vnr(t), t->comm);
> +		return -EPERM;
> +	}
> +
> +	/* verify that the task is frozen (unless self) */
> +	if (t != current && !frozen(t)) {
> +		__ckpt_write_err(ctx, "task %d (%s) is not frozen",
> +				 task_pid_vnr(t), t->comm);
> +		return -EBUSY;
> +	}
> +
> +	/* FIX: add support for ptraced tasks */
> +	if (task_ptrace(t)) {
> +		__ckpt_write_err(ctx, "task %d (%s) is ptraced",
> +				 task_pid_vnr(t), t->comm);
> +		return -EBUSY;
> +	}
> +
> +	return 0;
> +}
> +
> +static int get_container(struct ckpt_ctx *ctx, pid_t pid)
> +{
> +	struct task_struct *task = NULL;
> +	struct nsproxy *nsproxy = NULL;
> +	int ret;
> +
> +	ctx->root_pid = pid;
> +
> +	read_lock(&tasklist_lock);
> +	task = find_task_by_vpid(pid);
> +	if (task)
> +		get_task_struct(task);
> +	read_unlock(&tasklist_lock);
> +
> +	if (!task)
> +		return -ESRCH;
> +
> +	ret = may_checkpoint_task(ctx, task);
> +	if (ret) {
> +		ckpt_write_err(ctx, NULL);
> +		put_task_struct(task);
> +		return ret;
> +	}
> +
> +	rcu_read_lock();
> +	nsproxy = task_nsproxy(task);
> +	get_nsproxy(nsproxy);

Will oops if init is multi-threaded and thread group leader exited
(nsproxy = NULL). I need to think what to do, too.

> +	rcu_read_unlock();
> +
> +	ctx->root_task = task;
> +	ctx->root_nsproxy = nsproxy;
> +
> +	return 0;
> +}