Re: [PATCH] kill_something_info: don't take tasklist_lock for pid==-1 case

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From: Oleg Nesterov
Date: Tuesday, March 25, 2008 - 6:56 am

On 03/25, Atsushi Tsuji wrote:

Hmm. Yes, group_send_sig_info() doesn't need tasklist_lock. But we
take tasklist_lock to "freeze" the tasks list, so that we can't miss
a new forked process.

Same for __kill_pgrp_info(), we take tasklist to kill the whole group
"atomically".


However. Is it really needed? copy_process() returns -ERESTARTNOINTR
if signal_pending(), and the new task is always placed at the tail
of the list. Looks like nobody can escape the signal, at least fatal
or SIGSTOP.

If the signal is blocked/ignored or has a handler, we can miss a forked
child, but this looks OK, we can pretend it was forked after we dropped
tasklist_lock.

Note also that copy_process() does list_add_tail_rcu(p->tasks) under
->siglock, this means kill_something_info() must see the new childs
after group_send_sig_info() drops ->siglock.


Except: We don't send the signal to /sbin/init. This means that (say)
kill(-1, SIGKILL) can miss the task forked by init. Note that this
task could be forked even before we start kill_something_info(), but
without tasklist there is no guarantee we will see it on the ->tasks
list.

I think this is the only problem with this change.

Eric, Roland?

(Unfortunately, attach_pid() adds the task to the head of hlist, this
 means we can't avoid tasklist for __kill_pgrp_info).

Oleg.

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Messages in current thread:
Re: [PATCH] kill_something_info: don't take tasklist_lock ..., Oleg Nesterov, (Tue Mar 25, 6:56 am)
Re: [PATCH] kill_something_info: don't take tasklist_lock ..., Eric W. Biederman, (Tue May 20, 7:53 pm)
Re: [PATCH] kill_something_info: don't take tasklist_lock ..., Eric W. Biederman, (Tue May 20, 8:47 pm)
Re: [PATCH] kill_something_info: don't take tasklist_lock ..., Eric W. Biederman, (Wed May 28, 8:03 am)
Re: [PATCH] kill_something_info: don't take tasklist_lock ..., Eric W. Biederman, (Sat May 31, 4:55 pm)