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Date: Wed, 09 Jan 2008 17:02:28 +1100 From: Herbert Xu <herbert@...dor.apana.org.au> To: dada1@...mosbay.com (Eric Dumazet) Cc: davem@...emloft.net, dipankar@...ibm.com, netdev@...r.kernel.org Subject: Re: [IPV4] ROUTE: ip_rt_dump() is unecessary slow Eric Dumazet <dada1@...mosbay.com> wrote: > > Very good question, but honestly I really dont see why it was there at the > first place : It was there because someone went through this file and robotically replaced all conditional read barriers with rcu_dereference, even when it made zero sense. Basically you can add a conditional barrier either at the point where the pointer gets read, or where it gets derferenced. Previously we did the latter (except that the show function didn't have a barrier at all which is technically a bug though harmless in pratice). This patch moves it to the spot where it gets read which is also OK. > static struct rtable *rt_cache_get_next(struct seq_file *seq, struct rtable *r) > { > - struct rt_cache_iter_state *st = rcu_dereference(seq->private); > + struct rt_cache_iter_state *st = seq->private; > > - r = r->u.dst.rt_next; > + r = rcu_dereference(r->u.dst.rt_next); > while (!r) { > rcu_read_unlock_bh(); > if (--st->bucket < 0) > break; > rcu_read_lock_bh(); > - r = rt_hash_table[st->bucket].chain; > + r = rcu_dereference(rt_hash_table[st->bucket].chain); > } > return r; Slight optimisation: please move both barriers onto the return statement, i.e., return rcu_dereference(r); Cheers, -- Visit Openswan at http://www.openswan.org/ Email: Herbert Xu ~{PmV>HI~} <herbert@...dor.apana.org.au> Home Page: http://gondor.apana.org.au/~herbert/ PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt -- 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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