Re: [PATCH v4 03/10] kvm: dirty-ring: Fix race with vcpu creation

[Paolo Bonzini]

On 2/16/23 17:18, huangy81@chinatelecom.cn wrote:
> diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c
> index 9b26582655..47483cdfa0 100644
> --- a/accel/kvm/kvm-all.c
> +++ b/accel/kvm/kvm-all.c
> @@ -685,6 +685,15 @@ static uint32_t kvm_dirty_ring_reap_one(KVMState *s, 
> CPUState *cpu)
>       uint32_t ring_size = s->kvm_dirty_ring_size;
>       uint32_t count = 0, fetch = cpu->kvm_fetch_index;
>   
> +    /*
> +     * It's possible that we race with vcpu creation code where the vcpu is
> +     * put onto the vcpus list but not yet initialized the dirty ring
> +     * structures.  If so, skip it.
> +     */
> +    if (!cpu->created) {
> +        return 0;
> +    }
> +

Is there a lock that protects cpu->created?

If you don't want to use a lock you need to use qatomic_load_acquire
together with

diff --git a/softmmu/cpus.c b/softmmu/cpus.c
index fed20ffb5dd2..15b64e7f4592 100644
--- a/softmmu/cpus.c
+++ b/softmmu/cpus.c
@@ -525,7 +525,7 @@ void qemu_cond_timedwait_iothread(QemuCond *cond, int ms)
 /* signal CPU creation */
 void cpu_thread_signal_created(CPUState *cpu)
 {
-    cpu->created = true;
+    qatomic_store_release(&cpu->created, true);
     qemu_cond_signal(&qemu_cpu_cond);
 }
 

Paolo