linux-kernel - [PATCH V2 2/2] rbtree_latch: don't need to check seq when it found a node

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Date:   Fri, 15 May 2020 15:59:09 +0000
From:   Lai Jiangshan <laijs@...ux.alibaba.com>
To:     linux-kernel@...r.kernel.org
Cc:     Lai Jiangshan <laijs@...ux.alibaba.com>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Michel Lespinasse <walken@...gle.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Rik van Riel <riel@...hat.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: [PATCH V2 2/2] rbtree_latch: don't need to check seq when it found a node

latch_tree_find() should be protected by caller via RCU or so.
When it find a node in an attempt, the node must be a valid one
in RCU's point's of view even the tree is (being) updated with a
new node with the same key which is entirely subject to timing
anyway.

Cc: Paul E. McKenney <paulmck@...nel.org>
Cc: Oleg Nesterov <oleg@...hat.com>
Cc: Michel Lespinasse <walken@...gle.com>
Cc: Andrea Arcangeli <aarcange@...hat.com>
Cc: Rik van Riel <riel@...hat.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Signed-off-by: Lai Jiangshan <laijs@...ux.alibaba.com>
---
 include/linux/rbtree_latch.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/rbtree_latch.h b/include/linux/rbtree_latch.h
index 638942f53c0a..affc4b026d9b 100644
--- a/include/linux/rbtree_latch.h
+++ b/include/linux/rbtree_latch.h
@@ -245,7 +245,7 @@ latch_tree_find(void *key, struct latch_tree_root *root,
 	do {
 		seq = raw_read_seqcount_latch(&root->seq);
 		node = __lt_find(key, root, seq & 1, ops->comp);
-	} while (read_seqcount_retry(&root->seq, seq));
+	} while (!node && read_seqcount_retry(&root->seq, seq));
 
 	return node;
 }
-- 
2.20.1