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Date: Fri, 5 Dec 2014 16:03:26 +0000
From: KY Srinivasan <kys@...rosoft.com>
To: Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com>
CC: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"olaf@...fle.de" <olaf@...fle.de>,
"apw@...onical.com" <apw@...onical.com>,
"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: RE: [PATCH 1/1] mm: Fix a deadlock in the hotplug code
> -----Original Message-----
> From: Yasuaki Ishimatsu [mailto:isimatu.yasuaki@...fujitsu.com]
> Sent: Thursday, December 4, 2014 10:22 PM
> To: KY Srinivasan
> Cc: linux-kernel@...r.kernel.org; olaf@...fle.de; apw@...onical.com;
> linux-mm@...ck.org
> Subject: Re: [PATCH 1/1] mm: Fix a deadlock in the hotplug code
>
> (2014/12/03 5:46), K. Y. Srinivasan wrote:
> > Andy Whitcroft <apw@...onical.com> initially saw this deadlock. We
> > have seen this as well. Here is the original description of the
> > problem (and a potential solution) from Andy:
> >
> > https://lkml.org/lkml/2014/3/14/451
> >
> > Here is an excerpt from that mail:
> >
> > "We are seeing machines lockup with what appears to be an ABBA
> > deadlock in the memory hotplug system. These are from the 3.13.6 based
> Ubuntu kernels.
> > The hv_balloon driver is adding memory using add_memory() which takes
> > the hotplug lock, and then emits a udev event, and then attempts to
> > lock the sysfs device. In response to the udev event udev opens the
> > sysfs device and locks it, then attempts to grab the hotplug lock to online
> the memory.
> > This seems to be inverted nesting in the two cases, leading to the hangs
> below:
> >
> > [ 240.608612] INFO: task kworker/0:2:861 blocked for more than 120
> seconds.
> > [ 240.608705] INFO: task systemd-udevd:1906 blocked for more than 120
> seconds.
> >
> > I note that the device hotplug locking allows complete retries (via
> > ERESTARTSYS) and if we could detect this at the online stage it could
> > be used to get us out. But before I go down this road I wanted to
> > make sure I am reading this right. Or indeed if the hv_balloon driver
> > is just doing this wrong."
> >
> > This patch is based on Andy's analysis and suggestion.
>
> How about use lock_device_hotplug() before calling add_memory() in
> hv_mem_hot_add()?
> Commit 0f1cfe9d0d06 (mm/hotplug: remove stop_machine() from
> try_offline_node()) said:
>
> ---
> lock_device_hotplug() serializes hotplug & online/offline operations. The
> lock is held in common sysfs online/offline interfaces and ACPI hotplug
> code paths.
>
> And here are the code paths:
>
> - CPU & Mem online/offline via sysfs online
> store_online()->lock_device_hotplug()
>
> - Mem online via sysfs state:
> store_mem_state()->lock_device_hotplug()
>
> - ACPI CPU & Mem hot-add:
> acpi_scan_bus_device_check()->lock_device_hotplug()
>
> - ACPI CPU & Mem hot-delete:
> acpi_scan_hot_remove()->lock_device_hotplug()
> ---
>
> CPU & Memory online/offline/hotplug are serialized by
> lock_device_hotplug().
> So using lock_device_hotplug() solves the ABBA issue.
Thank you! I will make the necessary adjustments including exporting the relevant
Functions lock/unlock the device_hotplug lock.
Regards,
K. Y
>
> Thanks,
> Yasuaki Ishimatsu
>
> >
> > Signed-off-by: K. Y. Srinivasan <kys@...rosoft.com>
> > ---
> > mm/memory_hotplug.c | 24 +++++++++++++++++-------
> > 1 files changed, 17 insertions(+), 7 deletions(-)
> >
> > diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c index
> > 9fab107..e195269 100644
> > --- a/mm/memory_hotplug.c
> > +++ b/mm/memory_hotplug.c
> > @@ -104,19 +104,27 @@ void put_online_mems(void)
> >
> > }
> >
> > -static void mem_hotplug_begin(void)
> > +static int mem_hotplug_begin(bool trylock)
> > {
> > mem_hotplug.active_writer = current;
> >
> > memhp_lock_acquire();
> > for (;;) {
> > - mutex_lock(&mem_hotplug.lock);
> > + if (trylock) {
> > + if (!mutex_trylock(&mem_hotplug.lock)) {
> > + mem_hotplug.active_writer = NULL;
> > + return -ERESTARTSYS;
> > + }
> > + } else {
> > + mutex_lock(&mem_hotplug.lock);
> > + }
> > if (likely(!mem_hotplug.refcount))
> > break;
> > __set_current_state(TASK_UNINTERRUPTIBLE);
> > mutex_unlock(&mem_hotplug.lock);
> > schedule();
> > }
> > + return 0;
> > }
> >
> > static void mem_hotplug_done(void)
> > @@ -969,7 +977,9 @@ int __ref online_pages(unsigned long pfn, unsigned
> long nr_pages, int online_typ
> > int ret;
> > struct memory_notify arg;
> >
> > - mem_hotplug_begin();
> > + ret = mem_hotplug_begin(true);
> > + if (ret)
> > + return ret;
> > /*
> > * This doesn't need a lock to do pfn_to_page().
> > * The section can't be removed here because of the @@ -1146,7
> > +1156,7 @@ int try_online_node(int nid)
> > if (node_online(nid))
> > return 0;
> >
> > - mem_hotplug_begin();
> > + mem_hotplug_begin(false);
> > pgdat = hotadd_new_pgdat(nid, 0);
> > if (!pgdat) {
> > pr_err("Cannot online node %d due to NULL pgdat\n", nid);
> @@
> > -1236,7 +1246,7 @@ int __ref add_memory(int nid, u64 start, u64 size)
> > new_pgdat = !p;
> > }
> >
> > - mem_hotplug_begin();
> > + mem_hotplug_begin(false);
> >
> > new_node = !node_online(nid);
> > if (new_node) {
> > @@ -1684,7 +1694,7 @@ static int __ref __offline_pages(unsigned long
> start_pfn,
> > if (!test_pages_in_a_zone(start_pfn, end_pfn))
> > return -EINVAL;
> >
> > - mem_hotplug_begin();
> > + mem_hotplug_begin(false);
> >
> > zone = page_zone(pfn_to_page(start_pfn));
> > node = zone_to_nid(zone);
> > @@ -2002,7 +2012,7 @@ void __ref remove_memory(int nid, u64 start,
> u64
> > size)
> >
> > BUG_ON(check_hotplug_memory_range(start, size));
> >
> > - mem_hotplug_begin();
> > + mem_hotplug_begin(false);
> >
> > /*
> > * All memory blocks must be offlined before removing memory.
> > Check
> >
>
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