Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
From:   Lai Jiangshan <laijs@linux.alibaba.com>
To:     linux-kernel@vger.kernel.org
Cc:     Lai Jiangshan <laijs@linux.alibaba.com>,
        "Paul E . McKenney" <paulmck@kernel.org>,
        Oleg Nesterov <oleg@redhat.com>,
        Michel Lespinasse <walken@google.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Rik van Riel <riel@redhat.com>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Peter Zijlstra <peterz@infradead.org>
Subject: [PATCH V2 2/2] rbtree_latch: don't need to check seq when it found a node
Date:   Fri, 15 May 2020 15:59:09 +0000
Message-Id: <20200515155912.1713-2-laijs@linux.alibaba.com>
In-Reply-To: <20200515155912.1713-1-laijs@linux.alibaba.com>
References: <20200515150122.GY2957@hirez.programming.kicks-ass.net>
 <20200515155912.1713-1-laijs@linux.alibaba.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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@kernel.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Michel Lespinasse <walken@google.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Lai Jiangshan <laijs@linux.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