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From:   Lai Jiangshan <jiangshanlai+lkml@gmail.com>
Date:   Sat, 16 May 2020 12:52:47 +0800
Message-ID: <CAJhGHyB5BAR2k8OALutExW-Z1iOqjA1s5_vtsU2p2QNa2tE9eg@mail.gmail.com>
Subject: Re: [PATCH V2 2/2] rbtree_latch: don't need to check seq when it
 found a node
To:     Michel Lespinasse <walken@google.com>
Cc:     Lai Jiangshan <laijs@linux.alibaba.com>,
        LKML <linux-kernel@vger.kernel.org>,
        "Paul E . McKenney" <paulmck@kernel.org>,
        Oleg Nesterov <oleg@redhat.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Rik van Riel <riel@redhat.com>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Peter Zijlstra <peterz@infradead.org>
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On Sat, May 16, 2020 at 12:28 PM Michel Lespinasse <walken@google.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.