Re: [PATCH 1/2] LoongArch: KVM: Protect kvm_check_requests() with SRCU

[bibo mao <maobibo@xxxxxxxxxxx>]

On 2024/12/3 下午2:50, Huacai Chen wrote:
> When we enable lockdep we get such a warning:
>
>   =============================
>   WARNING: suspicious RCU usage
>   6.12.0-rc7+ #1891 Tainted: G        W
>   -----------------------------
>   include/linux/kvm_host.h:1043 suspicious rcu_dereference_check() usage!
>   other info that might help us debug this:
>   rcu_scheduler_active = 2, debug_locks = 1
>   1 lock held by qemu-system-loo/948:
>    #0: 90000001184a00a8 (&vcpu->mutex){+.+.}-{4:4}, at: kvm_vcpu_ioctl+0xf4/0xe20 [kvm]
>   stack backtrace:
>   CPU: 0 UID: 0 PID: 948 Comm: qemu-system-loo Tainted: G        W          6.12.0-rc7+ #1891
>   Tainted: [W]=WARN
>   Hardware name: Loongson Loongson-3A5000-7A1000-1w-CRB/Loongson-LS3A5000-7A1000-1w-CRB, BIOS vUDK2018-LoongArch-V2.0.0-prebeta9 10/21/2022
>   Stack : 0000000000000089 9000000005a0db9c 90000000071519c8 900000012c578000
>           900000012c57b920 0000000000000000 900000012c57b928 9000000007e53788
>           900000000815bcc8 900000000815bcc0 900000012c57b790 0000000000000001
>           0000000000000001 4b031894b9d6b725 0000000004dec000 90000001003299c0
>           0000000000000414 0000000000000001 000000000000002d 0000000000000003
>           0000000000000030 00000000000003b4 0000000004dec000 90000001184a0000
>           900000000806d000 9000000007e53788 00000000000000b4 0000000000000004
>           0000000000000004 0000000000000000 0000000000000000 9000000107baf600
>           9000000008916000 9000000007e53788 9000000005924778 0000000010000044
>           00000000000000b0 0000000000000004 0000000000000000 0000000000071c1d
>           ...
>   Call Trace:
>   [<9000000005924778>] show_stack+0x38/0x180
>   [<90000000071519c4>] dump_stack_lvl+0x94/0xe4
>   [<90000000059eb754>] lockdep_rcu_suspicious+0x194/0x240
>   [<ffff8000022143bc>] kvm_gfn_to_hva_cache_init+0xfc/0x120 [kvm]
>   [<ffff80000222ade4>] kvm_pre_enter_guest+0x3a4/0x520 [kvm]
>   [<ffff80000222b3dc>] kvm_handle_exit+0x23c/0x480 [kvm]
>
> Fix it by protecting kvm_check_requests() with SRCU.
>
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Huacai Chen <chenhuacai@xxxxxxxxxxx>
> ---
>   arch/loongarch/kvm/vcpu.c | 4 +++-
>   1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/arch/loongarch/kvm/vcpu.c b/arch/loongarch/kvm/vcpu.c
> index cab1818be68d..d18a4a270415 100644
> --- a/arch/loongarch/kvm/vcpu.c
> +++ b/arch/loongarch/kvm/vcpu.c
> @@ -240,7 +240,7 @@ static void kvm_late_check_requests(struct kvm_vcpu *vcpu)
>    */
>   static int kvm_enter_guest_check(struct kvm_vcpu *vcpu)
>   {
> -	int ret;
> +	int idx, ret;
>   
>   	/*
>   	 * Check conditions before entering the guest
> @@ -249,7 +249,9 @@ static int kvm_enter_guest_check(struct kvm_vcpu *vcpu)
>   	if (ret < 0)
>   		return ret;
>   
> +	idx = srcu_read_lock(&vcpu->kvm->srcu);
>   	ret = kvm_check_requests(vcpu);
> +	srcu_read_unlock(&vcpu->kvm->srcu, idx);
>   
>   	return ret;
>   }
How about adding rcu readlock with closest function 
kvm_update_stolen_time()?

 static int kvm_check_requests(struct kvm_vcpu *vcpu)
 {
+       int idx;
+
        if (!kvm_request_pending(vcpu))
                return RESUME_GUEST;

@@ -213,8 +215,11 @@ static int kvm_check_requests(struct kvm_vcpu *vcpu)
        if (kvm_dirty_ring_check_request(vcpu))
                return RESUME_HOST;

-       if (kvm_check_request(KVM_REQ_STEAL_UPDATE, vcpu))
+       if (kvm_check_request(KVM_REQ_STEAL_UPDATE, vcpu)) {
+               idx = srcu_read_lock(&vcpu->kvm->srcu);
                kvm_update_stolen_time(vcpu);
+               srcu_read_unlock(&vcpu->kvm->srcu, idx);
+       }

        return RESUME_GUEST;
 }

Both method look good to me.

Regards
Bibo Mao