linux-kernel - Re: [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 21:27:05 -0700
From:   Michel Lespinasse <walken@...gle.com>
To:     Lai Jiangshan <laijs@...ux.alibaba.com>
Cc:     linux-kernel@...r.kernel.org,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Rik van Riel <riel@...hat.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH V2 2/2] rbtree_latch: don't need to check seq when it
 found a node

On Fri, May 15, 2020 at 03:59:09PM +0000, Lai Jiangshan wrote:
> 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.

I'm not sure I buy this. Even if we get a valid node, is it the one we
were searching for ? I don't see how this could be guaranteed if the
read raced with a tree rebalancing.

-- 
Michel "Walken" Lespinasse
A program is never fully debugged until the last user dies.