Messages in this thread | | | Date | Thu, 5 Aug 2010 20:16:52 +0200 | From | Oleg Nesterov <> | Subject | Re: [PATCH 2/2] CRED: Fix __task_cred()'s lockdep check and banner comment |
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On 08/05, Linus Torvalds wrote: > > On Thu, Aug 5, 2010 at 12:19 AM, Eric W. Biederman > <ebiederm@xmission.com> wrote: > > > > No. When we send a signal to multiple processes it needs to be an > > atomic operation so that kill -KILL -pgrp won't let processes escape. > > It is what posix specifies, it is what real programs expect, and it > > is the useful semantic in userspace. > > Ok. However, in that case, it's not really about the whole list > traversal, it's a totally separate thing, and it's really sad that we > end up using the (rather hot) tasklist_lock for something like that. > With the dcache/inode locks basically going away, I think > tasklist_lock ends up being one of the few hot locks left. > > Wouldn't it be much nicer to: > - make it clear that all the "real" signal locking can rely on RCU > - use a separate per-pgrp lock that ends up being the one that gives > the signal _semantic_ meaning? > > That would automatically document why we get the lock too, which > certainly isn't clear from the code as-is. > > The per-pgrp lock might be something as simple as a silly hash that > just spreads out the process groups over some random number of simple > spinlocks.
I still think we can avoid the new lock and rely on RCU in kill_something_info() and kill_pgrp().
I am attaching my old email below. It talks about pgrp, however I think kill_something_info() is almost the same thing.
Oleg.
On 12/07, Eric W. Biederman wrote: > > Oleg Nesterov <oleg@redhat.com> writes: > > > On 12/05, Eric W. Biederman wrote: > >> > >> Atomically sending signal to every member of a process group, is the > >> big fly in the ointment I am aware of. Last time I looked I could > >> not see how to convert it rcu. > > > > I am not sure, but iirc we can do this lockless (under rcu_lock). > > We need to modify pid_link to use list_entry and attach_pid() should > > add the new task to the end. Of course we need more changes, but > > (again iirc) this is not too hard. > > The problem is that even adding to the end of the list, we could run > into a deleted entry and not see the new end of the list. > > Suppose when we start iterating the list we have: > > A -> B -> C -> D > > Then someone deletes some of the entries while we are iterating the list. > > A -> > B' -> C' -> D' > > We will continue on traversing through the deleted entries. > > Then someone adds a new entry to the end of the list. > > A-> N > > Since we are at B', C' or D' we will never see the new entry on the > end of the list.
Yes, but who can add the new entry?
Let's forget about setpgrp/etc for the moment, I think we have "races" with or without tasklist. Say, setpgrp() can add the new process to the already "killed" pgrp.
Then, I think the only important case is SIGKILL/SIGSTOP (or other signals which can't be blockes/ignored). We must kill/stop the entire pgrp, we must not race with fork() and miss a child.
In this case I _think_ rcu_read_lock() is enough,
rcu_read_lock()
list_for_each_entry_rcu(task, pid->tasks[PIDTYPE_PGID) group_send_sig_info(sig, task);
rcu_read_unlock();
except group_send_sig_info() can race with mt-exec, but this is simple to fix.
If we send a signal (not necessary SIGKILL) to a process P, we must see all childs which were forked by P, both send_signal() and copy_process() take the same ->siglock, we must see the result of list_add_tail_rcu(). And, after we sent SIGKILL/SIGSTOP, it can't fork the new child.
If list_for_each_entry() does not see the exited process P, this means we see the result of list_del_rcu(). But this also means we must the the result of the previous list_add_rcu().
IOW, fork+exit means list_add_rcu() + wmb() + list_del_rcu(), if we don't see the new entry on list, we must see the new one, right?
(I am ignoring the case when list_for_each_entry_rcu() sees a process P but lock_task_sighand(P) fails, I think this is the same as if we we missed P)
Now suppose a signal is blocked/ignored or has a handler. In this case we can miss a child, but I think this is OK, we can pretend the new child was forked after kill_pgrp() completes. Say, this child C was forked by some process P. We can miss C only if it was forked after we already sent the signal to P.
However. I do not pretend the reasoning above is "complete", and perhaps I missed something else.
> Additionally we have the other possibility that if a child is forking > we send the signal to the parent after the child forks away but before > the child joins whichever list we are walking, and we complete our > traversal without seeing the child.
Not sure I understand... But afaics this case is covered above. ->siglock should serialize this, copy_process() does attach_pid() under this lock.
Oleg.
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