Messages in this thread | | | Date | Tue, 25 Mar 2008 16:56:45 +0300 | From | Oleg Nesterov <> | Subject | Re: [PATCH] kill_something_info: don't take tasklist_lock for pid==-1 case |
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On 03/25, Atsushi Tsuji wrote: > > This patch avoid taking tasklist_lock in kill_something_info() when > the pid is -1. It can convert to rcu_read_lock() for this case because > group_send_sig_info() doesn't need tasklist_lock. > > This patch is for 2.6.25-rc5-mm1. > > Signed-off-by: Atsushi Tsuji <a-tsuji@bk.jp.nec.com> > --- > diff --git a/kernel/signal.c b/kernel/signal.c > index 3edbfd4..a888c58 100644 > --- a/kernel/signal.c > +++ b/kernel/signal.c > @@ -1089,14 +1089,16 @@ static int kill_something_info(int sig, struct > siginfo *info, int pid) > return ret; > } > > - read_lock(&tasklist_lock); > if (pid != -1) { > + read_lock(&tasklist_lock); > ret = __kill_pgrp_info(sig, info, > pid ? find_vpid(-pid) : task_pgrp(current)); > + read_unlock(&tasklist_lock); > } else { > int retval = 0, count = 0; > struct task_struct * p; > > + rcu_read_lock(); > for_each_process(p) { > if (p->pid > 1 && !same_thread_group(p, current)) { > int err = group_send_sig_info(sig, info, p); > @@ -1106,8 +1108,8 @@ static int kill_something_info(int sig, struct > siginfo *info, int pid) > } > } > ret = count ? retval : -ESRCH; > + rcu_read_unlock(); > } > - read_unlock(&tasklist_lock); > > return ret; > }
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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