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SubjectRe: [PATCH rc5-mm] pids: kill PIDTYPE_TGID
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Date
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
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