[PATCH 2/2] mm: mmap: annotate vm_lock_anon_vma locking properlyfor lockdep

From: Jiri Kosina
Date: Fri Jan 04 2013 - 19:35:38 EST

Commit 5a505085f04 ("mm/rmap: Convert the struct anon_vma::mutex to an
rwsem") turned anon_vma mutex to rwsem.

However, the properly annotated nested locking in mm_take_all_locks() has
been converted from

mutex_lock_nest_lock(&anon_vma->root->mutex, &mm->mmap_sem);



which is incomplete, and causes the false positive report from lockdep below.

Annotate the fact that mmap_sem is used as an outter lock to serialize taking
of all the anon_vma rwsems at once no matter the order, using the
down_write_nest_lock() primitive.

This patch fixes this lockdep report:

[ INFO: possible recursive locking detected ]
3.8.0-rc2-00036-g5f73896 #171 Not tainted
qemu-kvm/2315 is trying to acquire lock:
(&anon_vma->rwsem){+.+...}, at: [<ffffffff8115d549>] mm_take_all_locks+0x149/0x1b0

but task is already holding lock:
(&anon_vma->rwsem){+.+...}, at: [<ffffffff8115d549>] mm_take_all_locks+0x149/0x1b0

other info that might help us debug this:
Possible unsafe locking scenario:


*** DEADLOCK ***

May be due to missing lock nesting notation

4 locks held by qemu-kvm/2315:
#0: (&mm->mmap_sem){++++++}, at: [<ffffffff81177f1c>] do_mmu_notifier_register+0xfc/0x170
#1: (mm_all_locks_mutex){+.+...}, at: [<ffffffff8115d436>] mm_take_all_locks+0x36/0x1b0
#2: (&mapping->i_mmap_mutex){+.+...}, at: [<ffffffff8115d4c9>] mm_take_all_locks+0xc9/0x1b0
#3: (&anon_vma->rwsem){+.+...}, at: [<ffffffff8115d549>] mm_take_all_locks+0x149/0x1b0

stack backtrace:
Pid: 2315, comm: qemu-kvm Not tainted 3.8.0-rc2-00036-g5f73896 #171
Call Trace:
[<ffffffff810afea2>] print_deadlock_bug+0xf2/0x100
[<ffffffff810b1a76>] validate_chain+0x4f6/0x720
[<ffffffff810b1ff9>] __lock_acquire+0x359/0x580
[<ffffffff810b0e7d>] ? trace_hardirqs_on_caller+0x12d/0x1b0
[<ffffffff810b2341>] lock_acquire+0x121/0x190
[<ffffffff8115d549>] ? mm_take_all_locks+0x149/0x1b0
[<ffffffff815a12bf>] down_write+0x3f/0x70
[<ffffffff8115d549>] ? mm_take_all_locks+0x149/0x1b0
[<ffffffff8115d549>] mm_take_all_locks+0x149/0x1b0
[<ffffffff81177e88>] do_mmu_notifier_register+0x68/0x170
[<ffffffff81177fae>] mmu_notifier_register+0xe/0x10
[<ffffffffa04bd6ab>] kvm_create_vm+0x22b/0x330 [kvm]
[<ffffffffa04bd8a8>] kvm_dev_ioctl+0xf8/0x1a0 [kvm]
[<ffffffff811a45bd>] do_vfs_ioctl+0x9d/0x350
[<ffffffff815ad215>] ? sysret_check+0x22/0x5d
[<ffffffff811a4901>] sys_ioctl+0x91/0xb0
[<ffffffff815ad1e9>] system_call_fastpath+0x16/0x1b

Signed-off-by: Jiri Kosina <jkosina@xxxxxxx>
mm/mmap.c | 2 +-
1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/mm/mmap.c b/mm/mmap.c
index f54b235..35730ee 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -2886,7 +2886,7 @@ static void vm_lock_anon_vma(struct mm_struct *mm, struct anon_vma *anon_vma)
* The LSB of head.next can't change from under us
* because we hold the mm_all_locks_mutex.
- down_write(&anon_vma->root->rwsem);
+ down_write_nest_lock(&anon_vma->root->rwsem, &mm->mmap_sem);
* We can safely modify head.next after taking the
* anon_vma->root->rwsem. If some other vma in this mm shares
Jiri Kosina
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