KVM: eventfd: Fix false positive RCU usage warning

The splat below can be seen when running kvm-unit-test:

     =============================
     WARNING: suspicious RCU usage
     5.18.0-rc7 #5 Tainted: G          IOE
     -----------------------------
     /home/kernel/linux/arch/x86/kvm/../../../virt/kvm/eventfd.c:80 RCU-list traversed in non-reader section!!

     other info that might help us debug this:

     rcu_scheduler_active = 2, debug_locks = 1
     4 locks held by qemu-system-x86/35124:
      #0: ffff9725391d80b8 (&vcpu->mutex){+.+.}-{4:4}, at: kvm_vcpu_ioctl+0x77/0x710 [kvm]
      #1: ffffbd25cfb2a0b8 (&kvm->srcu){....}-{0:0}, at: vcpu_enter_guest+0xdeb/0x1900 [kvm]
      #2: ffffbd25cfb2b920 (&kvm->irq_srcu){....}-{0:0}, at: kvm_hv_notify_acked_sint+0x79/0x1e0 [kvm]
      #3: ffffbd25cfb2b920 (&kvm->irq_srcu){....}-{0:0}, at: irqfd_resampler_ack+0x5/0x110 [kvm]

     stack backtrace:
     CPU: 2 PID: 35124 Comm: qemu-system-x86 Tainted: G          IOE     5.18.0-rc7 #5
     Call Trace:
      <TASK>
      dump_stack_lvl+0x6c/0x9b
      irqfd_resampler_ack+0xfd/0x110 [kvm]
      kvm_notify_acked_gsi+0x32/0x90 [kvm]
      kvm_hv_notify_acked_sint+0xc5/0x1e0 [kvm]
      kvm_hv_set_msr_common+0xec1/0x1160 [kvm]
      kvm_set_msr_common+0x7c3/0xf60 [kvm]
      vmx_set_msr+0x394/0x1240 [kvm_intel]
      kvm_set_msr_ignored_check+0x86/0x200 [kvm]
      kvm_emulate_wrmsr+0x4f/0x1f0 [kvm]
      vmx_handle_exit+0x6fb/0x7e0 [kvm_intel]
      vcpu_enter_guest+0xe5a/0x1900 [kvm]
      kvm_arch_vcpu_ioctl_run+0x16e/0xac0 [kvm]
      kvm_vcpu_ioctl+0x279/0x710 [kvm]
      __x64_sys_ioctl+0x83/0xb0
      do_syscall_64+0x3b/0x90
      entry_SYSCALL_64_after_hwframe+0x44/0xae

resampler-list is protected by irq_srcu (see kvm_irqfd_assign), so fix
the false positive by using list_for_each_entry_srcu().
Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
Message-Id: <1652950153-12489-1-git-send-email-wanpengli@tencent.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

virt/kvm/eventfd.c

@@ -77,7 +77,8 @@ irqfd_resampler_ack(struct kvm_irq_ack_notifier *kian)
 
 	idx = srcu_read_lock(&kvm->irq_srcu);
 
-	list_for_each_entry_rcu(irqfd, &resampler->list, resampler_link)
+	list_for_each_entry_srcu(irqfd, &resampler->list, resampler_link,
+	    srcu_read_lock_held(&kvm->irq_srcu))
 		eventfd_signal(irqfd->resamplefd, 1);
 
 	srcu_read_unlock(&kvm->irq_srcu, idx);