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Date: Sat, 16 May 2020 12:52:47 +0800 From: Lai Jiangshan <jiangshanlai+lkml@...il.com> To: Michel Lespinasse <walken@...gle.com> Cc: Lai Jiangshan <laijs@...ux.alibaba.com>, LKML <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 Sat, May 16, 2020 at 12:28 PM Michel Lespinasse <walken@...gle.com> wrote: > > 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. It is valid because ops->comp() returns 0 and it should be the one we were searching for unless ops->comp() is wrong. The searched one could be possible just deleted, but it is still a legitimate searched result in RCU's point's of view. A tree rebalancing can cause a searching fails to find an existing target. This is the job of read_seqcount_retry() to tell you to retry. > > -- > Michel "Walken" Lespinasse > A program is never fully debugged until the last user dies.
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