linux-kernel - Re: [PATCH 2/6] locking: Introduce range reader/writer lock

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Date:   Mon, 15 May 2017 15:02:57 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Davidlohr Bueso <dave@...olabs.net>
Cc:     mingo@...nel.org, akpm@...ux-foundation.org, jack@...e.cz,
        kirill.shutemov@...ux.intel.com, ldufour@...ux.vnet.ibm.com,
        mhocko@...e.com, mgorman@...hsingularity.net,
        linux-kernel@...r.kernel.org, Davidlohr Bueso <dbueso@...e.de>
Subject: Re: [PATCH 2/6] locking: Introduce range reader/writer lock

On Mon, May 15, 2017 at 02:07:21AM -0700, Davidlohr Bueso wrote:

> + * Fairness and freedom of starvation are guaranteed by the lack of lock
> + * stealing, thus range locks depend directly on interval tree semantics.
> + * This is particularly for iterations, where the key for the rbtree is
> + * given by the interval's low endpoint,

So suppose the lock is held at [a,n], and I want to acquire [g,z], this
conflicts, therefore I wait.

While I wait, someone else comes in at [b,m], they too wait.

[a,n] is released, per ordering [b,m] acquires, I still wait.

[a,n] returns to wait.

[b,m] releases, does the iteration then restart and grant it to [a,n] or
will I (at [g,z]) finally acquire?

Since the code always does range_interval_tree_foreach() it would appear
to me [b,m] will always win and [g,z] could be made to wait
indefinitely (by always contending with another range that has a lower
starting point).

>                                          and duplicates are walked as it
> + * would an inorder traversal of the tree.

Are duplicates ordered in FIFO ? Afaict the above is free of actual
semantics.