Re: [PATCH rc5-mm] pids: kill PIDTYPE_TGID

[ebiederm@xmission.com (Eric W. Biederman)]

Oleg Nesterov <oleg@tv-sign.ru> writes:

> Changes:
> 	s/tgrp/thread_group/
>
> 	make it rcu-safe
>
> [PATCH rc5-mm] pids: kill PIDTYPE_TGID
>
> depends on pidhash-dont-count-idle-threads.patch
>
> This patch kills PIDTYPE_TGID pid_type thus saving one hash table
> in kernel/pid.c and speeding up subthreads create/destroy a bit.
> It is also a preparation for the further tref/pids rework.
>
> This patch adds 'struct list_head thread_group' to 'struct task_struct'
> instead.
>
> We don't detach group leader from PIDTYPE_PID namespace until another
> thread inherits it's ->pid == ->tgid, so we are safe wrt premature
> free_pidmap(->tgid) call.
>
> Currently there are no users of find_task_by_pid_type(PIDTYPE_TGID).
> Should the need arise, we can use find_task_by_pid()->group_leader.

Looks good.  I have one final nit.

> --- 2.6.16-rc5/kernel/fork.c~1_TGID	2006-03-09 00:17:24.000000000 +0300
> +++ 2.6.16-rc5/kernel/fork.c	2006-03-09 00:41:35.000000000 +0300
> @@ -1100,6 +1100,7 @@ static task_t *copy_process(unsigned lon
>  	 * We dont wake it up yet.
>  	 */
>  	p->group_leader = p;
> +	INIT_LIST_HEAD(&p->thread_group);
>  	INIT_LIST_HEAD(&p->ptrace_children);
>  	INIT_LIST_HEAD(&p->ptrace_list);
>  
> @@ -1153,7 +1154,9 @@ static task_t *copy_process(unsigned lon
>  			retval = -EAGAIN;
>  			goto bad_fork_cleanup_namespace;
>  		}
> +
>  		p->group_leader = current->group_leader;
> +		list_add_tail_rcu(&p->thread_group, &current->thread_group);
Can this be:
		list_add_tail_rcu(&p->thread_group, &current->group_leader->thread_group);

That way at least the odds of missing a new task_struct when doing an
rcu traversal are reduced almost to 0.

>  
>  		if (current->signal->group_stop_count > 0) {
>  			/*
> @@ -1201,7 +1204,6 @@ static task_t *copy_process(unsigned lon
>  			list_add_tail(&p->tasks, &init_task.tasks);
>  			__get_cpu_var(process_counts)++;
>  		}
> -		attach_pid(p, PIDTYPE_TGID, p->tgid);
>  		attach_pid(p, PIDTYPE_PID, p->pid);
>  		nr_threads++;
>  	}


Eric