[PATCH 2/3] mm, thp: fix possible circular locking dependency caused by sum_vm_event()

[Ebru Akagunduz]

Nested circular locking dependency detected by kernel robot (udevadm).

  udevadm/221 is trying to acquire lock:
   (&mm->mmap_sem){++++++}, at: [<ffffffff81262543>] __might_fault+0x83/0x150
  but task is already holding lock:
   (s_active#12){++++.+}, at: [<ffffffff813315ee>] kernfs_fop_write+0x8e/0x250
  which lock already depends on the new lock.

 Possible unsafe locking scenario:

 CPU0                    CPU1
 ----                    ----
 lock(s_active);
                         lock(cpu_hotplug.lock);
                         lock(s_active);
 lock(&mm->mmap_sem);

 the existing dependency chain (in reverse order) is:
 -> #2 (s_active#12){++++.+}:
        [<ffffffff8117da2c>] lock_acquire+0xac/0x180
        [<ffffffff8132f50a>] __kernfs_remove+0x2da/0x410
        [<ffffffff81330630>] kernfs_remove_by_name_ns+0x40/0x90
        [<ffffffff813339fb>] sysfs_remove_file_ns+0x2b/0x70
        [<ffffffff81ba8a16>] device_del+0x166/0x320
        [<ffffffff81ba943c>] device_destroy+0x3c/0x50
        [<ffffffff8105aa61>] cpuid_class_cpu_callback+0x51/0x70
        [<ffffffff81131ce9>] notifier_call_chain+0x59/0x190
        [<ffffffff81132749>] __raw_notifier_call_chain+0x9/0x10
        [<ffffffff810fe6b0>] __cpu_notify+0x40/0x90
        [<ffffffff810fe890>] cpu_notify_nofail+0x10/0x30
        [<ffffffff810fe8d7>] notify_dead+0x27/0x1e0
        [<ffffffff810fe273>] cpuhp_down_callbacks+0x93/0x190
        [<ffffffff82096062>] _cpu_down+0xc2/0x1e0
        [<ffffffff810ff727>] do_cpu_down+0x37/0x50
        [<ffffffff8110003b>] cpu_down+0xb/0x10
        [<ffffffff81038e4d>] _debug_hotplug_cpu+0x7d/0xd0
        [<ffffffff8435d6bb>] debug_hotplug_cpu+0xd/0x11
        [<ffffffff84352426>] do_one_initcall+0x138/0x1cf
        [<ffffffff8435270a>] kernel_init_freeable+0x24d/0x2de
        [<ffffffff8209533a>] kernel_init+0xa/0x120
        [<ffffffff820a7972>] ret_from_fork+0x22/0x50

 -> #1 (cpu_hotplug.lock#2){+.+.+.}:
        [<ffffffff8117da2c>] lock_acquire+0xac/0x180
        [<ffffffff820a20d1>] mutex_lock_nested+0x71/0x4c0
        [<ffffffff810ff526>] get_online_cpus+0x66/0x80
        [<ffffffff81246fb3>] sum_vm_event+0x23/0x1b0
        [<ffffffff81293768>] collapse_huge_page+0x118/0x10b0
        [<ffffffff81294c5d>] khugepaged+0x55d/0xe80
        [<ffffffff81130304>] kthread+0x134/0x1a0
        [<ffffffff820a7972>] ret_from_fork+0x22/0x50

 -> #0 (&mm->mmap_sem){++++++}:
        [<ffffffff8117bf61>] __lock_acquire+0x2861/0x31f0
        [<ffffffff8117da2c>] lock_acquire+0xac/0x180
        [<ffffffff8126257e>] __might_fault+0xbe/0x150
        [<ffffffff8133160f>] kernfs_fop_write+0xaf/0x250
        [<ffffffff812a8933>] __vfs_write+0x43/0x1a0
        [<ffffffff812a8d3a>] vfs_write+0xda/0x240
        [<ffffffff812a8f84>] SyS_write+0x44/0xa0
        [<ffffffff820a773c>] entry_SYSCALL_64_fastpath+0x1f/0xbd

This patch moves sum_vm_event() before taking down_write(&mm->mmap_sem)
to solve dependency lock.

Signed-off-by: Ebru Akagunduz <ebru.akagunduz@xxxxxxxxx>
---
 mm/huge_memory.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 91442a9..feee44c 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2451,6 +2451,9 @@ static void collapse_huge_page(struct mm_struct *mm,
 		goto out_nolock;
 	}
 
+	swap = get_mm_counter(mm, MM_SWAPENTS);
+	curr_allocstall = sum_vm_event(ALLOCSTALL);
+
 	/*
 	 * Prevent all access to pagetables with the exception of
 	 * gup_fast later hanlded by the ptep_clear_flush and the VM
@@ -2483,8 +2486,6 @@ static void collapse_huge_page(struct mm_struct *mm,
 		goto out;
 	}
 
-	swap = get_mm_counter(mm, MM_SWAPENTS);
-	curr_allocstall = sum_vm_event(ALLOCSTALL);
 	/*
 	 * Don't perform swapin readahead when the system is under pressure,
 	 * to avoid unnecessary resource consumption.
-- 
1.9.1