linux-kernel - Re: [PATCH 1/5] locking: Introduce range reader/writer lock

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Message-ID: <20170329091114.4fgotmjhh7u2v7ga@hirez.programming.kicks-ass.net>
Date:   Wed, 29 Mar 2017 11:11:14 +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, mhocko@...e.com,
        mgorman@...hsingularity.net, linux-kernel@...r.kernel.org,
        Davidlohr Bueso <dbueso@...e.de>
Subject: Re: [PATCH 1/5] locking: Introduce range reader/writer lock

On Mon, Mar 06, 2017 at 09:03:26PM -0800, Davidlohr Bueso wrote:
> +/*
> + * Implementation of read/write range locks.
> + *
> + * We keep interval tree of locked and to-be-locked ranges. When new range lock
> + * is requested, we add its interval to the tree and store number of intervals
> + * intersecting it to 'blocking_ranges'.
> + *
> + * When a range is unlocked, we again walk intervals that intersect with the
> + * unlocked one and decrement their 'blocking_ranges'. We wake up owner of any
> + * range lock whose 'blocking_ranges' drops to 0. For the shared case, the
> + * 'blocking_ranges' is only incremented if the intersecting range is not marked
> + * as a reader. 

Not a word about fairness and starvation... Such important topics for
lock primitives.

                   In order to mitigate some of the tree walk overhead for
> + * non-intersecting ranges, the tree's min/max values are maintained and consulted
> + * in O(1) in the fastpath.
> + */

Maybe that ought not be here, doesn't seem like a fundamental design
point and would thus be better suited for a comment near where this
implementation detail is located ?