[CRIU] [PATCH 4/5] pstree: try to find a free pid between busy pids

[Pavel Emelyanov]

On 02/19/2016 09:13 PM, Andrey Vagin wrote:
> From: Andrew Vagin <avagin at virtuozzo.com>
> 
> Currently our pid allocator returns max_pid++ and it can return a pid
> which is bigger than kernel.max_pid.
> 
> (00.821430)   5506: Error (cr-restore.c:1540): Pid 300 do not match expected 32768
> 
> https://jira.sw.ru/browse/PSBM-42202
> Signed-off-by: Andrew Vagin <avagin at virtuozzo.com>
> ---
>  criu/pstree.c | 93 +++++++++++++++++++++++++++++++++++------------------------
>  1 file changed, 56 insertions(+), 37 deletions(-)
> 
> diff --git a/criu/pstree.c b/criu/pstree.c
> index 1be8c48..61fd5d3 100644
> --- a/criu/pstree.c
> +++ b/criu/pstree.c
> @@ -331,8 +331,6 @@ err:
>  	return ret;
>  }
>  
> -static int max_pid = 0;
> -
>  static int prepare_pstree_for_shell_job(void)
>  {
>  	pid_t current_sid = getsid(getpid());
> @@ -379,8 +377,8 @@ static int prepare_pstree_for_shell_job(void)
>  			pi->sid = current_sid;
>  	}
>  
> -	max_pid = max((int)current_sid, max_pid);
> -	max_pid = max((int)current_gid, max_pid);
> +	insert_item(current_sid);
> +	insert_item(current_gid);

insert_item may fail.

>  
>  	return 0;
>  }
> @@ -459,11 +457,8 @@ static int read_pstree_image(void)
>  			break;
>  
>  		pi->pid.virt = e->pid;
> -		max_pid = max((int)e->pid, max_pid);
>  		pi->pgid = e->pgid;
> -		max_pid = max((int)e->pgid, max_pid);
>  		pi->sid = e->sid;
> -		max_pid = max((int)e->sid, max_pid);
>  		pi->pid.state = TASK_ALIVE;
>  
>  		if (e->ppid == 0) {
> @@ -543,11 +538,34 @@ static int read_pstree_image(void)
>  				goto err;
>  		}
>  	}
> +
>  err:
>  	close_image(img);
>  	return ret;
>  }
>  
> +#define RESERVED_PIDS           300
> +static int get_free_pid()
> +{
> +	static struct pstree_item *prev, *next;
> +
> +	if (prev == NULL)
> +		prev = rb_entry(rb_first(&root_rb), struct pstree_item, pid.node);
> +
> +	while (1) {
> +		pid_t pid;
> +		pid = prev->pid.virt + 1;
> +		pid = pid < RESERVED_PIDS ? RESERVED_PIDS + 1 : pid;
> +
> +		next = rb_entry(rb_next(&prev->pid.node), struct pstree_item, pid.node);
> +		if (&next->pid.node == NULL || next->pid.virt > pid)
> +			return pid;
> +		prev = next;
> +	}
> +
> +	return -1;
> +}
> +
>  static int prepare_pstree_ids(void)
>  {
>  	struct pstree_item *item, *child, *helper, *tmp;
> @@ -562,6 +580,7 @@ static int prepare_pstree_ids(void)
>  	 * reparented to init.
>  	 */
>  	list_for_each_entry(item, &root_item->children, sibling) {
> +		struct pstree_item *leader;
>  
>  		/*
>  		 * If a child belongs to the root task's session or it's
> @@ -571,15 +590,36 @@ static int prepare_pstree_ids(void)
>  		if (item->sid == root_item->sid || item->sid == item->pid.virt)
>  			continue;
>  
> -		helper = alloc_pstree_helper();
> -		if (helper == NULL)
> -			return -1;
> -		helper->sid = item->sid;
> -		helper->pgid = item->sid;
> -		helper->pid.virt = item->sid;
> -		helper->parent = root_item;
> -		helper->ids = root_item->ids;
> -		list_add_tail(&helper->sibling, &helpers);
> +		leader = insert_item(item->sid);
> +		if (leader->pid.state != TASK_UNDEF) {

This is changed behavior -- before the patch helper was allocated unconditionally,
now we _may_ switch to some existing task. Explain this change, please.

> +			pid_t pid;
> +
> +			pid = get_free_pid();
> +			if (pid < 0)
> +				break;
> +			helper = insert_item(pid);
> +			if (helper == NULL)
> +				return -1;
> +
> +			pr_info("Session leader %d\n", item->sid);
> +
> +			helper->sid = item->sid;
> +			helper->pgid = leader->pgid;
> +			helper->ids = leader->ids;
> +			helper->parent = leader;
> +			list_add(&helper->sibling, &leader->children);
> +
> +			pr_info("Attach %d to the task %d\n",
> +					helper->pid.virt, leader->pid.virt);
> +		} else {
> +			helper = leader;
> +			helper->sid = item->sid;
> +			helper->pgid = item->sid;
> +			helper->parent = root_item;
> +			helper->ids = root_item->ids;
> +			list_add_tail(&helper->sibling, &helpers);
> +		}
> +		init_pstree_helper(helper);
>  
>  		pr_info("Add a helper %d for restoring SID %d\n",
>  				helper->pid.virt, helper->sid);
> @@ -639,27 +679,6 @@ static int prepare_pstree_ids(void)
>  
>  			continue;
>  		}
> -
> -		pr_info("Session leader %d\n", item->sid);
> -
> -		/* Try to find helpers, who should be connected to the leader */
> -		list_for_each_entry(child, &helpers, sibling) {
> -			if (child->pid.state != TASK_HELPER)
> -				continue;
> -
> -			if (child->sid != item->sid)
> -				continue;
> -
> -			child->pgid = item->pgid;
> -			child->pid.virt = ++max_pid;
> -			child->parent = item;
> -			list_move(&child->sibling, &item->children);
> -
> -			pr_info("Attach %d to the task %d\n",
> -					child->pid.virt, item->pid.virt);
> -
> -			break;
> -		}
>  	}
>  
>  	/* All other helpers are session leaders for own sessions */
>