Messages in this thread Patch in this message | | | Date | Wed, 4 Aug 2010 15:17:49 +0200 | From | Oleg Nesterov <> | Subject | Re: [PATCH 2/2] CRED: Fix __task_cred()'s lockdep check and banner comment |
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On 08/03, Linus Torvalds wrote: > > On Tue, Aug 3, 2010 at 2:34 AM, David Howells <dhowells@redhat.com> wrote: > > > > A previous patch: > > > > commit 8f92054e7ca1d3a3ae50fb42d2253ac8730d9b2a > > Author: David Howells <dhowells@redhat.com> > > Date: Thu Jul 29 12:45:55 2010 +0100 > > Subject: CRED: Fix __task_cred()'s lockdep check and banner comment
I am not sure I understand this patch.
__task_cred() checks rcu_read_lock_held() || task_is_dead(), and task_is_dead(task) is ((task)->exit_state != 0).
OK, task_is_dead() is valid for, say, wait_task_zombie(). But wait_task_stopped() calls __task_cred(p) without rcu lock and p is alive. The code is correct, this thread can do nothing until we drop ->siglock.
> > fixed the lockdep checks on __task_cred(). This has shown up a place in the > > signalling code where a lock should be held - namely that > > check_kill_permission() requires its callers to hold the RCU lock. > > It's not just check_kill_permission(), is it? I thought we could do > the "for_each_process()" loops with just RCU, rather than holding the > whole tasklist_lock?
Yes, for_each_process() is rcu-safe by itself.
> So I _think_ that getting the RCU read-lock would > make it possible to get rid of the tasklist_lock in there too? At > least in kill_something_info().
As for kill_something_info(), I think yes. I even sent (iirc) the protoptype patch a long ago. We can't just remove tasklist, we should avoid the races fork/exit/exec in the kill(-1, SIG) case.
The same for kill_pgrp/__kill_pgrp_info(). We need tasklist to ensure that nobody in this group can escape the signal. This seems solveable too, it was even discussed a bit.
> > It's may be that it would be better to add RCU read lock calls in > > group_send_sig_info() only, around the call to check_kill_permission().
I must admit, at first glance changing check_kill_permission() to take rcu lock looks better to me.
> On the > > other hand, some of the callers are either holding the RCU read lock already, > > or have disabled interrupts,
Hmm. So, local_irq_disable() "officially" blocks rcu? It does in practice (unless I missed the new version of RCU), but, say, posix_timer_event() takes rcu_read_lock() exactly because I thought we shouldn't assume that irqs_disabled() acts as rcu_read_lock() ?
There are other examples of rcu_read_lock() under local_irq_disable().
> > --- a/kernel/exit.c > > +++ b/kernel/exit.c > > @@ -773,6 +773,7 @@ static void forget_original_parent(struct task_struct *father) > > > > exit_ptrace(father); > > > > + rcu_read_lock(); > > write_lock_irq(&tasklist_lock); > > reaper = find_new_reaper(father);
No, this doesn't look right. find_new_reaper() can drop tasklist and sleep.
Besides, this patch conflicts with the change in -mm tree. And imho this looks a bit as "action at a distance".
Oleg.
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