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Message-ID: <20100319115407.GG2894@linux.vnet.ibm.com> Date: Fri, 19 Mar 2010 04:54:07 -0700 From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> To: Eric Dumazet <eric.dumazet@...il.com> Cc: David Miller <davem@...emloft.net>, netdev <netdev@...r.kernel.org> Subject: Re: [PATCH] net: dev_getfirstbyhwtype() optimization On Fri, Mar 19, 2010 at 06:14:02AM +0100, Eric Dumazet wrote: > Le jeudi 18 mars 2010 à 19:32 -0700, Paul E. McKenney a écrit : > > On Thu, Mar 18, 2010 at 10:27:25PM +0100, Eric Dumazet wrote: > > > Use RCU to avoid RTNL use in dev_getfirstbyhwtype() > > > > > > Signed-off-by: Eric Dumazet <eric.dumazet@...il.com> > > > --- > > > diff --git a/net/core/dev.c b/net/core/dev.c > > > index 17b1686..0f2e9fc 100644 > > > --- a/net/core/dev.c > > > +++ b/net/core/dev.c > > > @@ -772,14 +772,17 @@ EXPORT_SYMBOL(__dev_getfirstbyhwtype); > > > > > > struct net_device *dev_getfirstbyhwtype(struct net *net, unsigned short type) > > > { > > > - struct net_device *dev; > > > + struct net_device *dev, *ret = NULL; > > > > > > - rtnl_lock(); > > > - dev = __dev_getfirstbyhwtype(net, type); > > > - if (dev) > > > - dev_hold(dev); > > > - rtnl_unlock(); > > > - return dev; > > > + rcu_read_lock(); > > > + for_each_netdev_rcu(net, dev) > > > + if (dev->type == type) { > > > + dev_hold(dev); > > > + ret = dev; > > > + break; > > > + } > > > + rcu_read_unlock(); > > > + return ret; > > > > Looks good, but I don't understand how it helps to introduce the > > local variable "ret". > > > > Thanks for reviewing Paul ! > > Not only it helps, its necessary :) > > for_each_netdev_rcu(net, dev) { > if (cond) { > dev_hold(dev); > break; > } > } > makes no guarantee dev is NULL if we hit the list end : > > > /** > * list_for_each_entry_rcu - iterate over rcu list of given type > * @pos: the type * to use as a loop cursor. > * @head: the head for your list. > * @member: the name of the list_struct within the struct. > * > * This list-traversal primitive may safely run concurrently with > * the _rcu list-mutation primitives such as list_add_rcu() > * as long as the traversal is guarded by rcu_read_lock(). > */ > #define list_for_each_entry_rcu(pos, head, member) \ > for (pos = list_entry_rcu((head)->next, typeof(*pos), member); \ > prefetch(pos->member.next), &pos->member != (head); \ > pos = list_entry_rcu(pos->member.next, typeof(*pos), member)) Right! Got it, thank you! Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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