On Wed, Aug 08, 2007 at 08:41:07PM +0400, Oleg Nesterov wrote: > This time Paul E. McKenney actually cc'ed, sorry for the extra > noise... > > On 08/08, Pavel Emelyanov wrote: > > > > When someone wants to deal with some other taks's namespaces > > it has to lock the task and then to get the desired namespace > > if the one exists. This is slow on read-only paths and may be > > impossible in some cases. > > > > E.g. Oleg recently noticed a race between unshare() and the > > (just sent for review) pid namespaces - when the task notifies > > the parent it has to know the parent's namespace, but taking > > the task_lock() is impossible there - the code is under write > > locked tasklist lock. > > > > On the other hand switching the namespace on task (daemonize) > > and releasing the namespace (after the last task exit) is rather > > rare operation and we can sacrifice its speed to solve the > > issues above. > > Still it is a bit sad we slow down process's exit. Perhaps I missed > some other ->nsproxy access, but can't we make a simpler patch? > > --- kernel/fork.c 2007-07-28 16:58:17.000000000 +0400 > +++ /proc/self/fd/0 2007-08-08 20:30:33.325216944 +0400 > @@ -1633,7 +1633,9 @@ asmlinkage long sys_unshare(unsigned lon > > if (new_nsproxy) { > old_nsproxy = current->nsproxy; > + read_lock(&tasklist_lock); > current->nsproxy = new_nsproxy; > + read_unlock(&tasklist_lock); > new_nsproxy = old_nsproxy; > } > > > This way ->nsproxy is stable under task_lock() or write_lock(tasklist). > > > +void switch_task_namespaces(struct task_struct *p, struct nsproxy *new) > > +{ > > + struct nsproxy *ns; > > + > > + might_sleep(); > > + > > + ns = p->nsproxy; > > + if (ns == new) > > + return; > > + > > + if (new) > > + get_nsproxy(new); > > + rcu_assign_pointer(p->nsproxy, new); > > + > > + if (ns && atomic_dec_and_test(&ns->count)) { > > + /* > > + * wait for others to get what they want from this > > + * nsproxy. cannot release this nsproxy via the > > + * call_rcu() since put_mnt_ns will want to sleep > > + */ > > + synchronize_rcu(); > > + free_nsproxy(ns); > > + } > > +} > > (I may be wrong, Paul cc'ed) > > This is correct with the current implementation of RCU, but strictly speaking, > we can't use synchronize_rcu() here, because write_lock_irq() doesn't imply > rcu_read_lock() in theory. Can you use synchronize_sched() instead? The synchronize_sched() primitive will wait until all preempt/irq-disable code sequences complete. Therefore, it would wait for all write_lock_irq() code sequences to complete. Does this work? Thanx, Paul _______________________________________________ Containers mailing list Containers@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/containers