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

Oleg Nesterov <oleg@xxxxxxxxxx> writes:

Eric W. Biederman wrote:

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);

Done.

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

Am I understand correctly? This change has effect when we are doing the
traversal starting from ->group_leader in a "best effort" manner lockless,
yes?

Yes. The important point for reasoning about it is that we have a fixed
point that we append or prepend to.

I think I have finally figured out the invariants we will need to
send signals using just the rcu_read_lock(). In that case we want to
traverse the list in such a way that we are guaranteed to never see
new entries. That gives us an atomic snapshot of the tasks to send
signals to, and it gives us a progress guarantee.

Now I need to look and see if any one has documented any rules
regarding atomicity of sending signals to a group of processes.


[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.

include/linux/pid.h | 1 -
include/linux/sched.h | 11 ++++++++---
kernel/exit.c | 10 +---------
kernel/fork.c | 4 +++-
4 files changed, 12 insertions(+), 14 deletions(-)

Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>

Acked-By: Eric Biederman <ebiederm@xxxxxxxxxxxx>
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