Messages in this thread |  | | | Date | Fri, 11 Nov 2005 21:22:13 -0800 | | From | "Paul E. McKenney" <> | | Subject | Re: [Lse-tech] Subject: [RFC][PATCH] Fix for unsafe notifier chain mechanism |
| |
On Fri, Nov 11, 2005 at 09:36:40PM -0500, Alan Stern wrote: > On Fri, 11 Nov 2005, Paul E. McKenney wrote: > > > On Fri, Nov 11, 2005 at 03:43:39PM -0800, Chandra Seetharaman wrote: > > > Hello, > > > > > > In 2.6.14, the notifier chains are unsafe. notifier_call_chain() walks through > > > the list of a call chain without any protection. > > > Looks pretty good! Some RCU-related review comments interspersed below. > > > > > Alan and I did think about changing the data structure to use list_head, but > > > deferred it (as a cleanup) as it is not directly tied with what Alan was > > > trying to fix. > > > > It would simplify the code... > > It would. It would also mean auditing every place in the kernel where a > notifier_block structure is defined. There are a _lot_ of them, and many > don't use C99 initializers or do initialize the link pointer. Chandra and > I decided it was best to leave this as a subsequent cleanup job, maybe > something suitable for kernel-janitors.
Fair enough by me!
> > > + down_write(&nh->rwsem); > > > + nl = &nh->head; > > > + while ((*nl) != NULL) { > > > + if (n->priority > (*nl)->priority) > > > + break; > > > + nl = &((*nl)->next); > > > + } > > > + rcu_assign_pointer(n->next, *nl); > > > > The above can simply be "n->next = *nl;". The reason is that this change > > of state is not visible to RCU readers until after the following statement, > > and it therefore need not be an RCU-reader-safe assignment. You only need > > to use rcu_assign_pointer() when the results of the assignment are > > immediately visible to RCU readers. > > Correct, the rcu call isn't really needed. It doesn't hurt perceptibly, > though, and part of the RCU documentation states: > > * ... More importantly, this > * call documents which pointers will be dereferenced by RCU read-side > * code. > > For that reason, I felt it was worth putting it in.
But the following statement does a much better job of documenting the pointer that is to be RCU-dereferenced. Duplicate documentation can be just as confusing as no documentation.
> > > + rcu_assign_pointer(*nl, n); > > > + up_write(&nh->rwsem); > > > + if (nh->type == ATOMIC_NOTIFIER) > > > + synchronize_rcu(); > > > > This "if" statement and the "synchronize_rcu()" are not needed. Nothing > > has been removed from the list, so nothing will be freed, so no need to > > wait for readers to get done. > > You're right. In an earlier form of the patch this call was left out, but > then it crept back in later. We can remove it.
Sounds good!
> > In contrast, the synchronize_rcu() in notifier_chain_unregister() -is- > > needed, since we need to free the removed element. > > > > + if (!nh->head) > > > + return ret; > > > + if (nh->type == ATOMIC_NOTIFIER) > > > + rcu_read_lock(); > > > + else > > > + down_read(&nh->rwsem); > > > > Is it possible for the value of nh->type to change? If so, there needs > > to be some additional mechanism to guard against such a change. However, > > if this field is constant, this code is just fine as is. > > nh->type is never supposed to change.
OK, then the code is fine as it is.
Thanx, Paul - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
|  |