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Subject[PATCH 3/3] Drivers: hv: balloon: fix deadlock between memory adding and onlining
Date
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():
...
[ 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:
...
[ 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

Solve the issue bu grabbing device_hotplug_lock before doing add_memory(). If
we do that, lock_device_hotplug_sysfs() will cause syscall retry which will
eventually succeed.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
---
drivers/hv/hv_balloon.c | 10 ++++++++++
1 file changed, 10 insertions(+)

diff --git a/drivers/hv/hv_balloon.c b/drivers/hv/hv_balloon.c
index b958ded..0af1aa2 100644
--- a/drivers/hv/hv_balloon.c
+++ b/drivers/hv/hv_balloon.c
@@ -592,9 +592,19 @@ static void hv_mem_hot_add(unsigned long start, unsigned long size,
dm_device.ha_waiting = true;

nid = memory_add_physaddr_to_nid(PFN_PHYS(start_pfn));
+
+ /*
+ * Grab hotplug lock as we'll be doing device_register() and we
+ * need to protect against someone (e.g. udev doing memory
+ * onlining) locking it before we're done.
+ */
+ lock_device_hotplug();
+
ret = add_memory(nid, PFN_PHYS((start_pfn)),
(HA_CHUNK << PAGE_SHIFT));

+ unlock_device_hotplug();
+
if (ret) {
pr_info("hot_add memory failed error is %d\n", ret);
if (ret == -EEXIST) {
--
1.9.3


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