Messages in this thread |  | | | Date | Sat, 12 Nov 2005 10:35:07 -0500 (EST) | | From | Alan Stern <> | | Subject | Re: [Lse-tech] Subject: [RFC][PATCH] Fix for unsafe notifier chain mechanism |
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On Fri, 11 Nov 2005, Paul E. McKenney wrote:
> > > > + 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.
It's not really duplicate documentation since _both_ pointers are to be RCU-dereferenced. But maybe you mean that only the second pointer can be RCU-dereferenced at the time the write occurs? I don't think that's what the documentation comment intended.
Alan Stern
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