Ryan Harper wrote:
> There is a race between when the vcpu thread issues a create ioctl and when
> apic_reset() gets called resulting in getting a badfd error.
>
>
The problem is indeed there, but the fix is wrong:
> main thread vcpu thread
> diff --git a/qemu/qemu-kvm.c b/qemu/qemu-kvm.c
> index 78127de..3513e8c 100644
> --- a/qemu/qemu-kvm.c
> +++ b/qemu/qemu-kvm.c
> @@ -31,7 +31,9 @@ extern int smp_cpus;
> static int qemu_kvm_reset_requested;
>
> pthread_mutex_t qemu_mutex = PTHREAD_MUTEX_INITIALIZER;
> +pthread_mutex_t vcpu_mutex = PTHREAD_MUTEX_INITIALIZER;
> pthread_cond_t qemu_aio_cond = PTHREAD_COND_INITIALIZER;
> +pthread_cond_t qemu_vcpuup_cond = PTHREAD_COND_INITIALIZER;
> __thread struct vcpu_info *vcpu;
>
> struct qemu_kvm_signal_table {
> @@ -369,6 +371,11 @@ static void *ap_main_loop(void *_env)
> sigfillset(&signals);
> sigprocmask(SIG_BLOCK, &signals, NULL);
> kvm_create_vcpu(kvm_context, env->cpu_index);
> + /* block until cond_wait occurs */
> + pthread_mutex_lock(&vcpu_mutex);
> + /* now we can signal */
> + pthread_cond_signal(&qemu_vcpuup_cond);
> + pthread_mutex_unlock(&vcpu_mutex);
> kvm_qemu_init_env(env);
> kvm_main_loop_cpu(env);
> return NULL;
> @@ -388,9 +395,10 @@ static void kvm_add_signal(struct qemu_kvm_signal_table
> *sigtab, int signum)
>
> void kvm_init_new_ap(int cpu, CPUState *env)
> {
> + pthread_mutex_lock(&vcpu_mutex);
> pthread_create(&vcpu_info[cpu].thread, NULL, ap_main_loop, env);
> - /* FIXME: wait for thread to spin up */
> - usleep(200);
> + pthread_cond_wait(&qemu_vcpuup_cond, &vcpu_mutex);
>
pthread_cond_wait() is never correct outside a loop. The signal may
arrive before wait is called.
The usual idiom is
while (condition is not fulfilled)
pthread_cond_wait();
I see you have something there to ensure we block, but please use the
right idiom.
> + pthread_mutex_unlock(&vcpu_mutex);
> }
>
Please reuse qemu_mutex for this, no need for a new one.
--
Do not meddle in the internals of kernels, for they are subtle and quick to
panic.
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