[1/4] rhashtable: use cmpxchg() in nested_table_alloc()
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Message ID 155349033954.1111.16843256359207016881.stgit@noble.brown
State Changes Requested
Delegated to: David Miller
Headers show
Series
  • Convert rhashtable to use bitlocks
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Commit Message

NeilBrown March 25, 2019, 5:05 a.m. UTC
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 as cmpxchg() can be used instead.

A future patch will replace the spinlocks by per-bucket bitlocks,
and then we won't be able to protect the slot pointer with a spinlock.

So replace rcu_assign_pointer() with cmpxchg() - which has equivalent
barrier properties.
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(-)

Patch
diff mbox series

diff --git a/lib/rhashtable.c b/lib/rhashtable.c
index f65e43fb1ff8..afe01a5048d7 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,