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From: NeilBrown <neilb@suse.com>
To: Thomas Graf <tgraf@suug.ch>, Herbert Xu <herbert@gondor.apana.org.au>
Cc: netdev@vger.kernel.org,
	"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
	linux-kernel@vger.kernel.org
Subject: [PATCH 1/3] rhashtable: use cmpxchg() in nested_table_alloc()
Date: Thu, 14 Mar 2019 16:05:28 +1100	[thread overview]
Message-ID: <155253992824.5022.10318044793639169265.stgit@noble.brown> (raw)
In-Reply-To: <155253979234.5022.1840929790507376038.stgit@noble.brown>

nested_table_alloc() relies on the fact that there is
at most one spinlock allocated for every slot in the top
level nested table, so it is not possible for two threads
to try to allocate the same table at the same time.

This assumption is a little fragile (it is not explicit) and is
unnecessary.  We can instead protect against
a race using cmpxchg() - if it the cmp fails, free the table
that was just allocated.

Signed-off-by: NeilBrown <neilb@suse.com>
---
 lib/rhashtable.c |    8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/lib/rhashtable.c b/lib/rhashtable.c
index 0a105d4af166..c983c0ee15d5 100644
--- a/lib/rhashtable.c
+++ b/lib/rhashtable.c
@@ -131,9 +131,11 @@ static union nested_table *nested_table_alloc(struct rhashtable *ht,
 			INIT_RHT_NULLS_HEAD(ntbl[i].bucket);
 	}
 
-	rcu_assign_pointer(*prev, ntbl);
-
-	return ntbl;
+	if (cmpxchg(prev, NULL, ntbl) == NULL)
+		return ntbl;
+	/* Raced with another thread. */
+	kfree(ntbl);
+	return rcu_dereference(*prev);
 }
 
 static struct bucket_table *nested_bucket_table_alloc(struct rhashtable *ht,



  parent reply	other threads:[~2019-03-14  5:06 UTC|newest]

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-03-14  5:05 [PATCH 0/3] Three rhashtable improvements NeilBrown
2019-03-14  5:05 ` [PATCH 3/3] rhashtable: rename rht_for_each*continue as *from NeilBrown
2019-03-15  5:49   ` Herbert Xu
2019-03-17  7:50   ` Miguel Ojeda
2019-03-18  0:15     ` NeilBrown
2019-03-14  5:05 ` [PATCH 2/3] rhashtable: don't hold lock on first table throughout insertion NeilBrown
2019-03-14 14:58   ` Paul E. McKenney
2019-03-15  5:47   ` Herbert Xu
2019-03-15  6:46     ` NeilBrown
2019-03-20  5:40       ` Herbert Xu
2019-03-14  5:05 ` NeilBrown [this message]
2019-03-15  5:10   ` [PATCH 1/3] rhashtable: use cmpxchg() in nested_table_alloc() Herbert Xu
2019-03-15  6:51     ` NeilBrown
2019-03-20  5:41       ` Herbert Xu

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