Re: [PATCH MANUALSEL 5.16 1/8] KVM: eventfd: Fix false positive RCU usage warning

[Paolo Bonzini]

On 2/9/22 19:56, Sasha Levin wrote:
> From: Hou Wenlong <houwenlong93@xxxxxxxxxxxxxxxxx>
>
> [ Upstream commit 6a0c61703e3a5d67845a4b275e1d9d7bc1b5aad7 ]

Acked-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>

Paolo

> Fix the following false positive warning:
>   =============================
>   WARNING: suspicious RCU usage
>   5.16.0-rc4+ #57 Not tainted
>   -----------------------------
>   arch/x86/kvm/../../../virt/kvm/eventfd.c:484 RCU-list traversed in non-reader section!!
>
>   other info that might help us debug this:
>
>   rcu_scheduler_active = 2, debug_locks = 1
>   3 locks held by fc_vcpu 0/330:
>    #0: ffff8884835fc0b0 (&vcpu->mutex){+.+.}-{3:3}, at: kvm_vcpu_ioctl+0x88/0x6f0 [kvm]
>    #1: ffffc90004c0bb68 (&kvm->srcu){....}-{0:0}, at: vcpu_enter_guest+0x600/0x1860 [kvm]
>    #2: ffffc90004c0c1d0 (&kvm->irq_srcu){....}-{0:0}, at: kvm_notify_acked_irq+0x36/0x180 [kvm]
>
>   stack backtrace:
>   CPU: 26 PID: 330 Comm: fc_vcpu 0 Not tainted 5.16.0-rc4+
>   Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.14.0-0-g155821a1990b-prebuilt.qemu.org 04/01/2014
>   Call Trace:
>    <TASK>
>    dump_stack_lvl+0x44/0x57
>    kvm_notify_acked_gsi+0x6b/0x70 [kvm]
>    kvm_notify_acked_irq+0x8d/0x180 [kvm]
>    kvm_ioapic_update_eoi+0x92/0x240 [kvm]
>    kvm_apic_set_eoi_accelerated+0x2a/0xe0 [kvm]
>    handle_apic_eoi_induced+0x3d/0x60 [kvm_intel]
>    vmx_handle_exit+0x19c/0x6a0 [kvm_intel]
>    vcpu_enter_guest+0x66e/0x1860 [kvm]
>    kvm_arch_vcpu_ioctl_run+0x438/0x7f0 [kvm]
>    kvm_vcpu_ioctl+0x38a/0x6f0 [kvm]
>    __x64_sys_ioctl+0x89/0xc0
>    do_syscall_64+0x3a/0x90
>    entry_SYSCALL_64_after_hwframe+0x44/0xae
>
> Since kvm_unregister_irq_ack_notifier() does synchronize_srcu(&kvm->irq_srcu),
> kvm->irq_ack_notifier_list is protected by kvm->irq_srcu. In fact,
> kvm->irq_srcu SRCU read lock is held in kvm_notify_acked_irq(), making it
> a false positive warning. So use hlist_for_each_entry_srcu() instead of
> hlist_for_each_entry_rcu().
>
> Reviewed-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> Signed-off-by: Hou Wenlong <houwenlong93@xxxxxxxxxxxxxxxxx>
> Message-Id: <f98bac4f5052bad2c26df9ad50f7019e40434512.1643265976.git.houwenlong.hwl@xxxxxxxxxxxx>
> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
> ---
>   virt/kvm/eventfd.c | 8 ++++----
>   1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/virt/kvm/eventfd.c b/virt/kvm/eventfd.c
> index 2ad013b8bde96..59b1dd4a549ee 100644
> --- a/virt/kvm/eventfd.c
> +++ b/virt/kvm/eventfd.c
> @@ -463,8 +463,8 @@ bool kvm_irq_has_notifier(struct kvm *kvm, unsigned irqchip, unsigned pin)
>   	idx = srcu_read_lock(&kvm->irq_srcu);
>   	gsi = kvm_irq_map_chip_pin(kvm, irqchip, pin);
>   	if (gsi != -1)
> -		hlist_for_each_entry_rcu(kian, &kvm->irq_ack_notifier_list,
> -					 link)
> +		hlist_for_each_entry_srcu(kian, &kvm->irq_ack_notifier_list,
> +					  link, srcu_read_lock_held(&kvm->irq_srcu))
>   			if (kian->gsi == gsi) {
>   				srcu_read_unlock(&kvm->irq_srcu, idx);
>   				return true;
> @@ -480,8 +480,8 @@ void kvm_notify_acked_gsi(struct kvm *kvm, int gsi)
>   {
>   	struct kvm_irq_ack_notifier *kian;
>   
> -	hlist_for_each_entry_rcu(kian, &kvm->irq_ack_notifier_list,
> -				 link)
> +	hlist_for_each_entry_srcu(kian, &kvm->irq_ack_notifier_list,
> +				  link, srcu_read_lock_held(&kvm->irq_srcu))
>   		if (kian->gsi == gsi)
>   			kian->irq_acked(kian);
>   }