linux-kernel - Re: [PATCH -mm 0/3] slab: cleanup mem hotplug synchronization

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Date:	Sun, 6 Apr 2014 21:46:45 +0400
From:	Vladimir Davydov <vdavydov@...allels.com>
To:	<akpm@...ux-foundation.org>
CC:	<linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>,
	<devel@...nvz.org>, Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Pekka Enberg <penberg@...nel.org>
Subject: Re: [PATCH -mm 0/3] slab: cleanup mem hotplug synchronization

On 04/06/2014 07:33 PM, Vladimir Davydov wrote:
> kmem_cache_{create,destroy,shrink} need to get a stable value of
> cpu/node online mask, because they init/destroy/access per-cpu/node
> kmem_cache parts, which can be allocated or destroyed on cpu/mem
> hotplug. To protect against cpu hotplug, these functions use
> {get,put}_online_cpus. However, they do nothing to synchronize with
> memory hotplug - taking the slab_mutex does not eliminate the
> possibility of race as described in patch 3.
>
> What we need there is something like get_online_cpus, but for memory. We
> already have lock_memory_hotplug, which serves for the purpose, but it's
> a bit of a hammer right now, because it's backed by a mutex. As a
> result, it imposes some limitations to locking order, which are not
> desirable, and can't be used just like get_online_cpus. I propose to
> turn this mutex into an rw semaphore, which will be taken for reading in
> lock_memory_hotplug and for writing in memory hotplug code (that's what
> patch 1 does).

This is absolutely wrong, because down_read cannot be nested inside
down/up_write critical section. Although it would work now, it could
result in deadlocks in future. Please ignore this set completely.

Actually we need to implement a recursive rw semaphore here, just like
cpu_hotplug_lock.

Sorry for the noise.

Thanks.
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