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Subject[RFC][PATCH 9/7] mm: fix mm_take_all_locks() locking order
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Subject: mm: fix mm_take_all_locks() locking order

Lockdep spotted:

=======================================================
[ INFO: possible circular locking dependency detected ]
2.6.27-rc1 #270
-------------------------------------------------------
qemu-kvm/2033 is trying to acquire lock:
(&inode->i_data.i_mmap_lock){----}, at: [<ffffffff802996cc>] mm_take_all_locks+0xc2/0xea

but task is already holding lock:
(&anon_vma->lock){----}, at: [<ffffffff8029967a>] mm_take_all_locks+0x70/0xea

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

-> #1 (&anon_vma->lock){----}:
[<ffffffff8025cd37>] __lock_acquire+0x11be/0x14d2
[<ffffffff8025d0a9>] lock_acquire+0x5e/0x7a
[<ffffffff804c655b>] _spin_lock+0x3b/0x47
[<ffffffff8029a2ef>] vma_adjust+0x200/0x444
[<ffffffff8029a662>] split_vma+0x12f/0x146
[<ffffffff8029bc60>] mprotect_fixup+0x13c/0x536
[<ffffffff8029c203>] sys_mprotect+0x1a9/0x21e
[<ffffffff8020c0db>] system_call_fastpath+0x16/0x1b
[<ffffffffffffffff>] 0xffffffffffffffff

-> #0 (&inode->i_data.i_mmap_lock){----}:
[<ffffffff8025ca54>] __lock_acquire+0xedb/0x14d2
[<ffffffff8025d397>] lock_release_non_nested+0x1c2/0x219
[<ffffffff8025d515>] lock_release+0x127/0x14a
[<ffffffff804c6403>] _spin_unlock+0x1e/0x50
[<ffffffff802995d9>] mm_drop_all_locks+0x7f/0xb0
[<ffffffff802a965d>] do_mmu_notifier_register+0xe2/0x112
[<ffffffff802a96a8>] mmu_notifier_register+0xe/0x10
[<ffffffffa0043b6b>] kvm_dev_ioctl+0x11e/0x287 [kvm]
[<ffffffff802bd0ca>] vfs_ioctl+0x2a/0x78
[<ffffffff802bd36f>] do_vfs_ioctl+0x257/0x274
[<ffffffff802bd3e1>] sys_ioctl+0x55/0x78
[<ffffffff8020c0db>] system_call_fastpath+0x16/0x1b
[<ffffffffffffffff>] 0xffffffffffffffff

other info that might help us debug this:

5 locks held by qemu-kvm/2033:
#0: (&mm->mmap_sem){----}, at: [<ffffffff802a95d0>] do_mmu_notifier_register+0x55/0x112
#1: (mm_all_locks_mutex){--..}, at: [<ffffffff8029963e>] mm_take_all_locks+0x34/0xea
#2: (&anon_vma->lock){----}, at: [<ffffffff8029967a>] mm_take_all_locks+0x70/0xea
#3: (&anon_vma->lock){----}, at: [<ffffffff8029967a>] mm_take_all_locks+0x70/0xea
#4: (&anon_vma->lock){----}, at: [<ffffffff8029967a>] mm_take_all_locks+0x70/0xea

stack backtrace:
Pid: 2033, comm: qemu-kvm Not tainted 2.6.27-rc1 #270

Call Trace:
[<ffffffff8025b7c7>] print_circular_bug_tail+0xb8/0xc3
[<ffffffff8025ca54>] __lock_acquire+0xedb/0x14d2
[<ffffffff80259bb1>] ? add_lock_to_list+0x7e/0xad
[<ffffffff8029967a>] ? mm_take_all_locks+0x70/0xea
[<ffffffff8029967a>] ? mm_take_all_locks+0x70/0xea
[<ffffffff8025d397>] lock_release_non_nested+0x1c2/0x219
[<ffffffff802996cc>] ? mm_take_all_locks+0xc2/0xea
[<ffffffff802996cc>] ? mm_take_all_locks+0xc2/0xea
[<ffffffff8025b202>] ? trace_hardirqs_on_caller+0x4d/0x115
[<ffffffff802995d9>] ? mm_drop_all_locks+0x7f/0xb0
[<ffffffff8025d515>] lock_release+0x127/0x14a
[<ffffffff804c6403>] _spin_unlock+0x1e/0x50
[<ffffffff802995d9>] mm_drop_all_locks+0x7f/0xb0
[<ffffffff802a965d>] do_mmu_notifier_register+0xe2/0x112
[<ffffffff802a96a8>] mmu_notifier_register+0xe/0x10
[<ffffffffa0043b6b>] kvm_dev_ioctl+0x11e/0x287 [kvm]
[<ffffffff8033f9f2>] ? file_has_perm+0x83/0x8e
[<ffffffff802bd0ca>] vfs_ioctl+0x2a/0x78
[<ffffffff802bd36f>] do_vfs_ioctl+0x257/0x274
[<ffffffff802bd3e1>] sys_ioctl+0x55/0x78
[<ffffffff8020c0db>] system_call_fastpath+0x16/0x1b

Which the locking hierarchy in mm/rmap.c confirms as 'valid'.

Although I don't think there are any users of these two locks that don't
hold the mmap_sem, therefore the nesting is strictly ok, but since we
already have an established order, we might as well respect it.

Fix this by first taking all the mapping->i_mmap_lock instances and then
take all anon_vma->lock instances.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
---
mm/mmap.c | 10 ++++++++--
1 file changed, 8 insertions(+), 2 deletions(-)

Index: linux-2.6/mm/mmap.c
===================================================================
--- linux-2.6.orig/mm/mmap.c
+++ linux-2.6/mm/mmap.c
@@ -2358,11 +2358,17 @@ int mm_take_all_locks(struct mm_struct *
for (vma = mm->mmap; vma; vma = vma->vm_next) {
if (signal_pending(current))
goto out_unlock;
- if (vma->anon_vma)
- vm_lock_anon_vma(mm, vma->anon_vma);
if (vma->vm_file && vma->vm_file->f_mapping)
vm_lock_mapping(mm, vma->vm_file->f_mapping);
}
+
+ for (vma = mm->mmap; vma; vma = vma->vm_next) {
+ if (signal_pending(current))
+ goto out_unlock;
+ if (vma->anon_vma)
+ vm_lock_anon_vma(mm, vma->anon_vma);
+ }
+
ret = 0;

out_unlock:



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