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Message-ID: <alpine.DEB.2.02.1402051217520.5616@chino.kir.corp.google.com>
Date: Wed, 5 Feb 2014 12:29:27 -0800 (PST)
From: David Rientjes <rientjes@...gle.com>
To: Nathan Zimmer <nzimmer@....com>
cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
Andrew Morton <akpm@...ux-foundation.org>,
Tang Chen <tangchen@...fujitsu.com>,
Wen Congyang <wency@...fujitsu.com>,
Toshi Kani <toshi.kani@...com>,
Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com>,
Xishi Qiu <qiuxishi@...wei.com>,
Cody P Schafer <cody@...ux.vnet.ibm.com>,
"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
Jiang Liu <liuj97@...il.com>, Hedi Berriche <hedi@....com>,
Mike Travis <travis@....com>
Subject: Re: [RFC] Move the memory_notifier out of the memory_hotplug lock
On Wed, 5 Feb 2014, Nathan Zimmer wrote:
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 62a0cd1..a3cbd14 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -985,12 +985,12 @@ int __ref online_pages(unsigned long pfn, unsigned long nr_pages, int online_typ
> if (need_zonelists_rebuild)
> zone_pcp_reset(zone);
> mutex_unlock(&zonelists_mutex);
> + unlock_memory_hotplug();
> printk(KERN_DEBUG "online_pages [mem %#010llx-%#010llx] failed\n",
> (unsigned long long) pfn << PAGE_SHIFT,
> (((unsigned long long) pfn + nr_pages)
> << PAGE_SHIFT) - 1);
> memory_notify(MEM_CANCEL_ONLINE, &arg);
> - unlock_memory_hotplug();
> return ret;
> }
>
> @@ -1016,9 +1016,10 @@ int __ref online_pages(unsigned long pfn, unsigned long nr_pages, int online_typ
>
> writeback_set_ratelimit();
>
> + unlock_memory_hotplug();
> +
> if (onlined_pages)
> memory_notify(MEM_ONLINE, &arg);
> - unlock_memory_hotplug();
>
> return 0;
> }
That looks a little problematic, what happens if a nid is being brought
online and a registered callback does something like allocate resources
for the arg->status_change_nid and the above two hunks of this patch end
up racing?
Before, a registered callback would be guaranteed to see either a
MEMORY_CANCEL_ONLINE or MEMORY_ONLINE after it has already done
MEMORY_GOING_ONLINE.
With your patch, we could race and see one cpu doing MEMORY_GOING_ONLINE,
another cpu doing MEMORY_GOING_ONLINE, and then MEMORY_ONLINE and
MEMORY_CANCEL_ONLINE in either order.
So I think this patch will break most registered callbacks that actually
depend on lock_memory_hotplug(), it's a coarse lock for that reason.
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