Messages in this thread | | | Subject | Re: [RFC][PATCH 0/5] Signal scalability series | From | Matt Fleming <> | Date | Tue, 04 Oct 2011 09:56:17 +0100 |
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On Mon, 2011-10-03 at 15:16 +0200, Oleg Nesterov wrote: > Why do we have? Usually SIGCONT is ignored. But this doesn't matter, > SIGCONT acts at the sending time. > > If SIGCONT is sent - the process must not stop. Since we drop the lock > we can't guarantee this.
OK, I see, thanks.
> > > May be do_signal_stop() does something special? At first flance it doesn't. > > > But wait, it does while_each_thread() under ->ctrl_lock, why this is safe? > > > > Why is it not safe? What scenario are you thinking of where that isn't > > safe? > > This series doesn't add ->ctrl_lock into copy_process/__unhash_process > or I misread the patches. This means we can't trust >thread_group list.
*facepalm*
Arrrrggghh! This is why I complain about sighand->siglock protecting too much, I didn't even _REALISE_ it protected the ->thread_group list. Thanks for pointing that out, Oleg!
> Even this is safe (say, we can rely on rcu), we can't calculate > ->group_stop_count correctly. In particular, without ->siglock we can > race with exit_signals() which sets PF_EXITING. Note that PF_EXITING > check in task_set_jobctl_pending() is important.
Ah, I think it was these lines that confused me into thinking ->ctrl_lock wasn't required around PF_EXITING,
void exit_signals(struct task_struct *tsk) { int group_stop = 0; sigset_t unblocked;
if (thread_group_empty(tsk) || signal_group_exit(tsk->signal)) { tsk->flags |= PF_EXITING; return; }
But I guess that's safe because either we're the only thread in the group or the group is already going to exit?
-- Matt Fleming, Intel Open Source Technology Center
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