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Date: Thu, 14 Mar 2019 16:05:28 +1100 From: NeilBrown <neilb@...e.com> To: Thomas Graf <tgraf@...g.ch>, Herbert Xu <herbert@...dor.apana.org.au> Cc: netdev@...r.kernel.org, "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>, linux-kernel@...r.kernel.org Subject: [PATCH 1/3] rhashtable: use cmpxchg() in nested_table_alloc() 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@...e.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,
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