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SubjectRe: [PATCH 0/3] memory_hotplug: hyperv: fix deadlock between memory adding and onlining
On Wed, 11 Feb 2015, David Rientjes wrote:

> > If newly added memory is brought online with e.g. udev rule:
> > SUBSYSTEM=="memory", ACTION=="add", ATTR{state}="online"
> > the following deadlock is observed (and easily reproducable):
> >
> > First participant, worker thread doing add_memory():
> >
> > [ 724.948846] kworker/0:1 D ffff88000412f9c8 13248 27 2 0x00000000
> > [ 724.973543] Workqueue: events hot_add_req [hv_balloon]
> > [ 724.991736] ffff88000412f9c8 0000000000000000 ffff88003fa1dc30 00000000000151c0
> > [ 725.019725] 0000000000000246 ffff88000412ffd8 00000000000151c0 ffff88003a77a4e0
> > [ 725.046486] ffff88003fa1dc30 00000001032a6000 ffff88003a7ca838 ffff88003a7ca898
> > [ 725.072969] Call Trace:
> > [ 725.082690] [<ffffffff81aac0a9>] schedule_preempt_disabled+0x29/0x70
> > [ 725.103799] [<ffffffff81aae33b>] mutex_lock_nested+0x14b/0x470
> > [ 725.122367] [<ffffffff815ed773>] ? device_attach+0x23/0xb0
> > [ 725.140992] [<ffffffff815ed773>] device_attach+0x23/0xb0
> > [ 725.159131] [<ffffffff815ecba0>] bus_probe_device+0xb0/0xe0
> > [ 725.177055] [<ffffffff815ea693>] device_add+0x443/0x650
> > [ 725.195558] [<ffffffff815ea8be>] device_register+0x1e/0x30
> > [ 725.213133] [<ffffffff81601790>] init_memory_block+0xd0/0xf0
> > [ 725.231533] [<ffffffff816018f1>] register_new_memory+0xb1/0xd0
> > [ 725.250769] [<ffffffff81a961cf>] __add_pages+0x13f/0x250
> > [ 725.269642] [<ffffffff81063770>] ? arch_add_memory+0x70/0xf0
> > [ 725.288764] [<ffffffff81063770>] arch_add_memory+0x70/0xf0
> > [ 725.306117] [<ffffffff81a95f8f>] add_memory+0xef/0x1f0
> > [ 725.322466] [<ffffffffa00293af>] hot_add_req+0x33f/0xf90 [hv_balloon]
> > [ 725.342777] [<ffffffff8109509f>] process_one_work+0x1df/0x4e0
> > [ 725.361459] [<ffffffff8109502d>] ? process_one_work+0x16d/0x4e0
> > [ 725.380390] [<ffffffff810954bb>] worker_thread+0x11b/0x450
> > [ 725.397684] [<ffffffff810953a0>] ? process_one_work+0x4e0/0x4e0
> > [ 725.416533] [<ffffffff8109ac33>] kthread+0xf3/0x110
> > [ 725.433372] [<ffffffff8109ab40>] ? kthread_create_on_node+0x240/0x240
> > [ 725.453749] [<ffffffff81ab1dfc>] ret_from_fork+0x7c/0xb0
> > [ 725.470994] [<ffffffff8109ab40>] ? kthread_create_on_node+0x240/0x240
> > [ 725.491469] 6 locks held by kworker/0:1/27:
> > [ 725.505037] #0: ("events"){......}, at: [<ffffffff8109502d>] process_one_work+0x16d/0x4e0
> > [ 725.533370] #1: ((&dm_device.ha_wrk.wrk)){......}, at: [<ffffffff8109502d>] process_one_work+0x16d/0x4e0
> > [ 725.565580] #2: (mem_hotplug.lock){......}, at: [<ffffffff811e6525>] mem_hotplug_begin+0x5/0x80
> > [ 725.594369] #3: (mem_hotplug.lock#2){......}, at: [<ffffffff811e656f>] mem_hotplug_begin+0x4f/0x80
> > [ 725.628554] #4: (mem_sysfs_mutex){......}, at: [<ffffffff81601873>] register_new_memory+0x33/0xd0
> > [ 725.658519] #5: (&dev->mutex){......}, at: [<ffffffff815ed773>] device_attach+0x23/0xb0
> >
> > Second participant, udev:
> >
> > [ 725.750889] systemd-udevd D ffff88003b94fc68 14016 888 530 0x00000004
> > [ 725.773767] ffff88003b94fc68 0000000000000000 ffff8800034949c0 00000000000151c0
> > [ 725.798332] ffffffff8210d980 ffff88003b94ffd8 00000000000151c0 ffff880037a69270
> > [ 725.822841] ffff8800034949c0 0000000100000001 ffff8800034949c0 ffffffff81ff2b48
> > [ 725.849184] Call Trace:
> > [ 725.858987] [<ffffffff81aac0a9>] schedule_preempt_disabled+0x29/0x70
> > [ 725.879231] [<ffffffff81aae33b>] mutex_lock_nested+0x14b/0x470
> > [ 725.897860] [<ffffffff811e656f>] ? mem_hotplug_begin+0x4f/0x80
> > [ 725.916698] [<ffffffff811e656f>] mem_hotplug_begin+0x4f/0x80
> > [ 725.935064] [<ffffffff811e6525>] ? mem_hotplug_begin+0x5/0x80
> > [ 725.953464] [<ffffffff81a9631b>] online_pages+0x3b/0x520
> > [ 725.971542] [<ffffffff815eb0b3>] ? device_online+0x23/0xa0
> > [ 725.989207] [<ffffffff81601524>] memory_subsys_online+0x64/0xc0
> > [ 726.008513] [<ffffffff815eb0fd>] device_online+0x6d/0xa0
> > [ 726.025579] [<ffffffff816012eb>] store_mem_state+0x5b/0xe0
> > [ 726.043400] [<ffffffff815e8258>] dev_attr_store+0x18/0x30
> > [ 726.060506] [<ffffffff8127a808>] sysfs_kf_write+0x48/0x60
> > [ 726.077940] [<ffffffff81279d1b>] kernfs_fop_write+0x13b/0x1a0
> > [ 726.099416] [<ffffffff811f9f67>] vfs_write+0xb7/0x1f0
> > [ 726.115748] [<ffffffff811fabf8>] SyS_write+0x58/0xd0
> > [ 726.131933] [<ffffffff81ab1ea9>] system_call_fastpath+0x12/0x17
> > [ 726.150691] 7 locks held by systemd-udevd/888:
> > [ 726.165044] #0: (sb_writers#3){......}, at: [<ffffffff811fa063>] vfs_write+0x1b3/0x1f0
> > [ 726.192422] #1: (&of->mutex){......}, at: [<ffffffff81279c46>] kernfs_fop_write+0x66/0x1a0
> > [ 726.220289] #2: (s_active#60){......}, at: [<ffffffff81279c4e>] kernfs_fop_write+0x6e/0x1a0
> > [ 726.249382] #3: (device_hotplug_lock){......}, at: [<ffffffff815e9c15>] lock_device_hotplug_sysfs+0x15/0x50
> > [ 726.281901] #4: (&dev->mutex){......}, at: [<ffffffff815eb0b3>] device_online+0x23/0xa0
> > [ 726.308619] #5: (mem_hotplug.lock){......}, at: [<ffffffff811e6525>] mem_hotplug_begin+0x5/0x80
> > [ 726.337994] #6: (mem_hotplug.lock#2){......}, at: [<ffffffff811e656f>] mem_hotplug_begin+0x4f/0x80
> >
> > In short: onlining grabs device lock and then tries to do mem_hotplug_begin()
> > while add_memory() is between mem_hotplug_begin() and mem_hotplug_done() and it
> > tries grabbing device lock.
> >
> > To my understanding ACPI memory hotplug doesn't have the same issue as
> > device_hotplug_lock is being grabbed when the ACPI device is added.
> >
> > Solve the issue by grabbing device_hotplug_lock before doing add_memory(). If
> > we do that, lock_device_hotplug_sysfs() will cause syscall retry which will
> > eventually succeed. To support the change we need to export lock_device_hotplug/
> > unlock_device_hotplug. This approach can be completely wrong though.
>
> Saying the approach could be completely wrong doesn't inspire a lot of
> confidence. I assume this output is from the hung task detector, is there
> any other lockdep output that suggests there's a possible deadlock?
>

Ok, I looked at this and the problem is that kworker/0 is onlining memory
and serializes memory hot-add with mem_hotplug_begin() before registering
the new memory block. This is the appropriate lock ordering, we want to
do mem_hotplug_begin() before device_lock(dev) which takes dev->mutex
since we must disallow concurrent hot-add events from looking up the same
memory block.

The issue only arises when systemd-udevd takes device_lock(dev) to
transition a memory block from offline to online and
memory_subsys_online() callbacks require mem_hotplug_begin().

Understanding this is pretty simple: in the kworker/0 case, we must create
a memory block and add the range by probing; in the systemd-udevd case, we
already have a memory block and need to transition its state.

Your approach to resolve this dependency is to serialize all of this with
device_hotplug_lock so that only one thread can be doing
mem_hotplug_begin() -> device_lock() or device_lock() ->
mem_hotplug_begin() at a time. I don't think resolving a locking
dependency is appropriate by just serializing them with another lock;
rather, I think the solution is to truly make one lock depend on the other
for memory hotplug.

I already mentioned that the appropriate lock ordering is
mem_hotplug_begin() -> device_lock() since we can't possibly know the
device that we are onlining for probe (we must create a new device, it
didn't exist before probe).

So all we need to do is require store_mem_state() to take
mem_hotplug_begin() before doing device_online() and requiring all
memory_subsys_online() callbacks to assume the protection, which they
already require anyway.

Could you try this patch out instead of your series? I did it for memory
hot-remove as well just to simplify the dependency, but it would also be
possible to just do mem_hotplug_begin() when onlining since we already
have the memory block registered for hot-remove. It's simpler this way.
---
diff --git a/drivers/base/memory.c b/drivers/base/memory.c
--- a/drivers/base/memory.c
+++ b/drivers/base/memory.c
@@ -219,6 +219,7 @@ static bool pages_correctly_reserved(unsigned long start_pfn)
/*
* MEMORY_HOTPLUG depends on SPARSEMEM in mm/Kconfig, so it is
* OK to have direct references to sparsemem variables in here.
+ * Must already be protected by mem_hotplug_begin().
*/
static int
memory_block_action(unsigned long phys_index, unsigned long action, int online_type)
@@ -286,6 +287,7 @@ static int memory_subsys_online(struct device *dev)
if (mem->online_type < 0)
mem->online_type = MMOP_ONLINE_KEEP;

+ /* Already under protection of mem_hotplug_begin() */
ret = memory_block_change_state(mem, MEM_ONLINE, MEM_OFFLINE);

/* clear online_type */
@@ -328,17 +330,19 @@ store_mem_state(struct device *dev,
goto err;
}

+ /*
+ * Memory hotplug needs to hold mem_hotplug_begin() for probe to find
+ * the correct memory block to online before doing device_online(dev),
+ * which will take dev->mutex. Take the lock early to prevent an
+ * inversion, memory_subsys_online() callbacks will be implemented by
+ * assuming it's already protected.
+ */
+ mem_hotplug_begin();
+
switch (online_type) {
case MMOP_ONLINE_KERNEL:
case MMOP_ONLINE_MOVABLE:
case MMOP_ONLINE_KEEP:
- /*
- * mem->online_type is not protected so there can be a
- * race here. However, when racing online, the first
- * will succeed and the second will just return as the
- * block will already be online. The online type
- * could be either one, but that is expected.
- */
mem->online_type = online_type;
ret = device_online(&mem->dev);
break;
@@ -349,6 +353,8 @@ store_mem_state(struct device *dev,
ret = -EINVAL; /* should never happen */
}

+ mem_hotplug_done();
+
err:
unlock_device_hotplug();

diff --git a/include/linux/memory_hotplug.h b/include/linux/memory_hotplug.h
--- a/include/linux/memory_hotplug.h
+++ b/include/linux/memory_hotplug.h
@@ -192,6 +192,9 @@ extern void get_page_bootmem(unsigned long ingo, struct page *page,
void get_online_mems(void);
void put_online_mems(void);

+void mem_hotplug_begin(void);
+void mem_hotplug_done(void);
+
#else /* ! CONFIG_MEMORY_HOTPLUG */
/*
* Stub functions for when hotplug is off
@@ -231,6 +234,9 @@ static inline int try_online_node(int nid)
static inline void get_online_mems(void) {}
static inline void put_online_mems(void) {}

+static inline void mem_hotplug_begin(void) {}
+static inline void mem_hotplug_done(void) {}
+
#endif /* ! CONFIG_MEMORY_HOTPLUG */

#ifdef CONFIG_MEMORY_HOTREMOVE
diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -104,7 +104,7 @@ void put_online_mems(void)

}

-static void mem_hotplug_begin(void)
+void mem_hotplug_begin(void)
{
mem_hotplug.active_writer = current;

@@ -119,7 +119,7 @@ static void mem_hotplug_begin(void)
}
}

-static void mem_hotplug_done(void)
+void mem_hotplug_done(void)
{
mem_hotplug.active_writer = NULL;
mutex_unlock(&mem_hotplug.lock);
@@ -959,6 +959,7 @@ static void node_states_set_node(int node, struct memory_notify *arg)
}


+/* Must be protected by mem_hotplug_begin() */
int __ref online_pages(unsigned long pfn, unsigned long nr_pages, int online_type)
{
unsigned long flags;
@@ -969,7 +970,6 @@ int __ref online_pages(unsigned long pfn, unsigned long nr_pages, int online_typ
int ret;
struct memory_notify arg;

- mem_hotplug_begin();
/*
* This doesn't need a lock to do pfn_to_page().
* The section can't be removed here because of the
@@ -977,21 +977,20 @@ int __ref online_pages(unsigned long pfn, unsigned long nr_pages, int online_typ
*/
zone = page_zone(pfn_to_page(pfn));

- ret = -EINVAL;
if ((zone_idx(zone) > ZONE_NORMAL ||
online_type == MMOP_ONLINE_MOVABLE) &&
!can_online_high_movable(zone))
- goto out;
+ return -EINVAL;

if (online_type == MMOP_ONLINE_KERNEL &&
zone_idx(zone) == ZONE_MOVABLE) {
if (move_pfn_range_left(zone - 1, zone, pfn, pfn + nr_pages))
- goto out;
+ return -EINVAL;
}
if (online_type == MMOP_ONLINE_MOVABLE &&
zone_idx(zone) == ZONE_MOVABLE - 1) {
if (move_pfn_range_right(zone, zone + 1, pfn, pfn + nr_pages))
- goto out;
+ return -EINVAL;
}

/* Previous code may changed the zone of the pfn range */
@@ -1007,7 +1006,7 @@ int __ref online_pages(unsigned long pfn, unsigned long nr_pages, int online_typ
ret = notifier_to_errno(ret);
if (ret) {
memory_notify(MEM_CANCEL_ONLINE, &arg);
- goto out;
+ return ret;
}
/*
* If this zone is not populated, then it is not in zonelist.
@@ -1031,7 +1030,7 @@ int __ref online_pages(unsigned long pfn, unsigned long nr_pages, int online_typ
(((unsigned long long) pfn + nr_pages)
<< PAGE_SHIFT) - 1);
memory_notify(MEM_CANCEL_ONLINE, &arg);
- goto out;
+ return ret;
}

zone->present_pages += onlined_pages;
@@ -1061,9 +1060,8 @@ int __ref online_pages(unsigned long pfn, unsigned long nr_pages, int online_typ

if (onlined_pages)
memory_notify(MEM_ONLINE, &arg);
-out:
- mem_hotplug_done();
- return ret;
+
+ return 0;
}
#endif /* CONFIG_MEMORY_HOTPLUG_SPARSE */

@@ -1684,21 +1682,18 @@ static int __ref __offline_pages(unsigned long start_pfn,
if (!test_pages_in_a_zone(start_pfn, end_pfn))
return -EINVAL;

- mem_hotplug_begin();
-
zone = page_zone(pfn_to_page(start_pfn));
node = zone_to_nid(zone);
nr_pages = end_pfn - start_pfn;

- ret = -EINVAL;
if (zone_idx(zone) <= ZONE_NORMAL && !can_offline_normal(zone, nr_pages))
- goto out;
+ return -EINVAL;

/* set above range as isolated */
ret = start_isolate_page_range(start_pfn, end_pfn,
MIGRATE_MOVABLE, true);
if (ret)
- goto out;
+ return ret;

arg.start_pfn = start_pfn;
arg.nr_pages = nr_pages;
@@ -1791,7 +1786,6 @@ repeat:
writeback_set_ratelimit();

memory_notify(MEM_OFFLINE, &arg);
- mem_hotplug_done();
return 0;

failed_removal:
@@ -1801,12 +1795,10 @@ failed_removal:
memory_notify(MEM_CANCEL_OFFLINE, &arg);
/* pushback to free area */
undo_isolate_page_range(start_pfn, end_pfn, MIGRATE_MOVABLE);
-
-out:
- mem_hotplug_done();
return ret;
}

+/* Must be protected by mem_hotplug_begin() */
int offline_pages(unsigned long start_pfn, unsigned long nr_pages)
{
return __offline_pages(start_pfn, start_pfn + nr_pages, 120 * HZ);

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