linux-kernel - Re: [PATCH v4] drivers/base/memory.c: cache blocks in radix tree to accelerate lookup

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Date:   Thu, 9 Jan 2020 14:00:04 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Scott Cheloha <cheloha@...ux.vnet.ibm.com>
Cc:     linux-kernel@...r.kernel.org,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        David Hildenbrand <david@...hat.com>, nathanl@...ux.ibm.com,
        ricklind@...ux.vnet.ibm.com, mhocko@...e.com,
        Scott Cheloha <cheloha@...ux.ibm.com>
Subject: Re: [PATCH v4] drivers/base/memory.c: cache blocks in radix tree to
 accelerate lookup

On Thu,  9 Jan 2020 15:25:16 -0600 Scott Cheloha <cheloha@...ux.vnet.ibm.com> wrote:

> Searching for a particular memory block by id is an O(n) operation
> because each memory block's underlying device is kept in an unsorted
> linked list on the subsystem bus.
> 
> We can cut the lookup cost to O(log n) if we cache the memory blocks in
> a radix tree.  With a radix tree cache in place both memory subsystem
> initialization and memory hotplug run palpably faster on systems with a
> large number of memory blocks.
> 
> ...
>
> @@ -56,6 +57,13 @@ static struct bus_type memory_subsys = {
>  	.offline = memory_subsys_offline,
>  };
>  
> +/*
> + * Memory blocks are cached in a local radix tree to avoid
> + * a costly linear search for the corresponding device on
> + * the subsystem bus.
> + */
> +static RADIX_TREE(memory_blocks, GFP_KERNEL);

What protects this tree from racy accesses?