linux-kernel - Re: [PATCH] mm/memory-hotplug: Switch locking to a percpu rwsem

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Message-ID: <20170703163204.GE11848@dhcp22.suse.cz>
Date:   Mon, 3 Jul 2017 18:32:04 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Andrey Ryabinin <aryabinin@...tuozzo.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>
Subject: Re: [PATCH] mm/memory-hotplug: Switch locking to a percpu rwsem

On Fri 30-06-17 12:15:21, Thomas Gleixner wrote:
[...]
> Sure. Just to make you to mull over more stuff, find below the patch which
> moves all of this to use the cpuhotplug lock.
> 
> Thanks,
> 
> 	tglx
> 
> 8<--------------------
> Subject: mm/memory-hotplug: Use cpu hotplug lock
> From: Thomas Gleixner <tglx@...utronix.de>
> Date: Thu, 29 Jun 2017 16:30:00 +0200
> 
> Most place which take the memory hotplug lock take the cpu hotplug lock as
> well. Avoid the double locking and use the cpu hotplug lock for both.

Hmm, I am usually not a fan of locks conflating because it is then less
clear what the lock actually protects. Memory and cpu hotplugs should
be largely independent so I am not sure this patch simplify things a
lot. It is nice to see few lines go away but I am little bit worried
that we will enventually develop a separate locking again in future for
some weird memory hotplug usecases.
 
> Not-Yet-Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
[...]
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
[...]
> @@ -2138,7 +2114,7 @@ void __ref remove_memory(int nid, u64 st
>  
>  	try_offline_node(nid);
>  
> -	mem_hotplug_done();
> +	cpus_write_lock();

unlock you meant here, right?

-- 
Michal Hocko
SUSE Labs