From 82202b6f9712706b25cdd6dcb51975483ef08baf Mon Sep 17 00:00:00 2001
From: "Isaac J. Manjarres" <isaacm@codeaurora.org>
Date: Wed, 22 Jul 2020 13:29:42 -0700
Subject: [PATCH] mm/memory_hotplug: Fix deadlock in try_online_one_block()

There is a possible scenario where the system can deadlock
itself when it is low on memory, and attempts to online
a memory block. Consider the following scenario:

 -A thread in userspace onlines a memory block through sysfs-
 state_store() /* Acquires the device_hotplug_lock */
 device_online()
 memory_subsys_online()
 memory_block_change_state()
 memory_block_action()
 online_pages()
 memory_notify(MEM_GOING_ONLINE, ...)
 online_page_ext()
 alloc_page_ext()
 __alloc_pages_nodemask(GFP_KERNEL, ...)
 /* System enters into a critically low memory state */
 out_of_memory()
 try_online_one_block() /* Invokes lock_device_hotplug() and deadlocks */

Fix deadlock by introducing and using trylock_device_hotplug()
instead of lock_device_hotplug().

Change-Id: I52bc8075ae713c50e3c381216f3616ce8366cf31
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>
---
 drivers/base/core.c    | 5 +++++
 include/linux/device.h | 1 +
 mm/memory_hotplug.c    | 3 ++-
 3 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/drivers/base/core.c b/drivers/base/core.c
index 7c30401a33c8..8e4ce59b950a 100644
--- a/drivers/base/core.c
+++ b/drivers/base/core.c
@@ -1321,6 +1321,11 @@ int lock_device_hotplug_sysfs(void)
 	return restart_syscall();
 }
 
+int trylock_device_hotplug(void)
+{
+	return mutex_trylock(&device_hotplug_lock);
+}
+
 #ifdef CONFIG_SCHED_WALT
 void lock_device_hotplug_assert(void)
 {
diff --git a/include/linux/device.h b/include/linux/device.h
index 7b5ac82614ff..e75a6a4c7e81 100644
--- a/include/linux/device.h
+++ b/include/linux/device.h
@@ -1592,6 +1592,7 @@ static inline bool device_supports_offline(struct device *dev)
 extern void lock_device_hotplug(void);
 extern void unlock_device_hotplug(void);
 extern int lock_device_hotplug_sysfs(void);
+extern int trylock_device_hotplug(void);
 #ifdef CONFIG_SCHED_WALT
 extern void lock_device_hotplug_assert(void);
 #endif
diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index 7dd64ab7c8ad..11467734f120 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -1025,7 +1025,8 @@ bool try_online_one_block(int nid)
 	unsigned long zone_start, zone_size;
 	bool onlined_block = false;
 
-	lock_device_hotplug();
+	if (!trylock_device_hotplug())
+		return false;
 
 	zone_start = PFN_PHYS(zone->zone_start_pfn);
 	zone_size = zone->spanned_pages << PAGE_SHIFT;