netdev - rhashtable: Remove unnecessary wmb for future

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Message-ID: <20151208090904.GA17605@gondor.apana.org.au>
Date:	Tue, 8 Dec 2015 17:09:04 +0800
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	"David S. Miller" <davem@...emloft.net>,
	Thomas Graf <tgraf@...g.ch>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Josh Triplett <josh@...htriplett.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	netdev@...r.kernel.org
Subject: rhashtable: Remove unnecessary wmb for future_tbl

The patch 9497df88ab5567daa001829051c5f87161a81ff0 ("rhashtable:
Fix reader/rehash race") added a pair of barriers.  In fact the
wmb is superfluous because every subsequent write to the old or
new hash table uses rcu_assign_pointer, which itself carriers a
full barrier prior to the assignment.

Therefore we may remove the explicit wmb.

Signed-off-by: Herbert Xu <herbert@...dor.apana.org.au>

diff --git a/lib/rhashtable.c b/lib/rhashtable.c
index 1c624db..d1f94b4 100644
--- a/lib/rhashtable.c
+++ b/lib/rhashtable.c
@@ -232,9 +232,6 @@ static int rhashtable_rehash_attach(struct rhashtable *ht,
 	 */
 	rcu_assign_pointer(old_tbl->future_tbl, new_tbl);
 
-	/* Ensure the new table is visible to readers. */
-	smp_wmb();
-
 	spin_unlock_bh(old_tbl->locks);
 
 	return 0;
-- 
Email: Herbert Xu <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
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