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SubjectRe: [BUG] kvm: dereference srcu-protected pointer without srcu_read_lock() held
Marcelo Tosatti wrote:
> On Mon, Apr 19, 2010 at 01:08:29PM +0300, Avi Kivity wrote:
>> On 04/19/2010 12:58 PM, Lai Jiangshan wrote:
>>> Applied the patch I just sent and let CONFIG_PROVE_RCU=y,
>>> we can got the following dmesg. And we found that it is
>>> because some codes in KVM dereferences srcu-protected pointer without
>>> srcu_read_lock() held or update-side lock held.
>>> It is not hard to fix, the problem is that:
>>> Where is the most proper place to put a srcu_read_lock()?
>>> I can not determine the answer, so I report this bug
>>> instead of fixing it.
>> I think the else branch in complete_pio() should work. Marcelo?
>> Longer term I'd like to see the lock taken at the high levels
>> (ioctls, in virt/kvm) and dropped only for guest entry and when we
>> explicitly sleep (hlt emulation).
>> Note: complete_pio() is gone in the current code.
> Yes, this was fixed by 7fb2ea1e6.

Applied the patch I sent yesterday and let CONFIG_PROVE_RCU=y
I can get the following dmesg.

Under very simple test, these is no complaint from PROVE_RCU
after this patch applied.

More test or reviewing of code are need in future.

Subject: [PATCH] kvm: add missing srcu_read_lock()

I got this dmesg due to srcu_read_lock() is missing in

[ INFO: suspicious rcu_dereference_check() usage. ]
arch/x86/kvm/x86.h:72 invoked rcu_dereference_check() without protection!

other info that might help us debug this:

rcu_scheduler_active = 1, debug_locks = 0
2 locks held by qemu-system-x86/3100:
#0: (rcu_read_lock){.+.+..}, at: [<ffffffff810d73dc>] __mmu_notifier_release+0x38/0xdf
#1: (&(&kvm->mmu_lock)->rlock){+.+...}, at: [<ffffffffa0130a6a>] kvm_mmu_zap_all+0x21/0x5e [kvm]

stack backtrace:
Pid: 3100, comm: qemu-system-x86 Not tainted 2.6.34-rc3-22949-gbc8a97a-dirty #2
Call Trace:
[<ffffffff8106afd9>] lockdep_rcu_dereference+0xaa/0xb3
[<ffffffffa0123a89>] unalias_gfn+0x56/0xab [kvm]
[<ffffffffa0119600>] gfn_to_memslot+0x16/0x25 [kvm]
[<ffffffffa012ffca>] gfn_to_rmap+0x17/0x6e [kvm]
[<ffffffffa01300c1>] rmap_remove+0xa0/0x19d [kvm]
[<ffffffffa0130649>] kvm_mmu_zap_page+0x109/0x34d [kvm]
[<ffffffffa0130a7e>] kvm_mmu_zap_all+0x35/0x5e [kvm]
[<ffffffffa0122870>] kvm_arch_flush_shadow+0x16/0x22 [kvm]
[<ffffffffa01189e0>] kvm_mmu_notifier_release+0x15/0x17 [kvm]
[<ffffffff810d742c>] __mmu_notifier_release+0x88/0xdf
[<ffffffff810d73dc>] ? __mmu_notifier_release+0x38/0xdf
[<ffffffff81040848>] ? exit_mm+0xe0/0x115
[<ffffffff810c2cb0>] exit_mmap+0x2c/0x17e
[<ffffffff8103c472>] mmput+0x2d/0xd4
[<ffffffff81040870>] exit_mm+0x108/0x115

Signed-off-by: Lai Jiangshan <>
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index a5dfea1..a6d639d 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -341,7 +341,11 @@ static void kvm_mmu_notifier_release(struct mmu_notifier *mn,
struct mm_struct *mm)
struct kvm *kvm = mmu_notifier_to_kvm(mn);
+ int idx;
+ idx = srcu_read_lock(&kvm->srcu);
+ srcu_read_unlock(&kvm->srcu, idx);

static const struct mmu_notifier_ops kvm_mmu_notifier_ops = {

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