Ulrich Drepper wrote:
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> Ryan Harper wrote:
>
>> @@ -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_mutex_unlock(&vcpu_mutex);
>>
>
> And something is very wrong here. The pattern for using a condvar is
>
> 1 take mutex
>
> 2 check condition
>
> 3 if condition is not fulfilled
> 3a call cond_wait
> 3b when returning, go back to step 2
>
> 4 unlock mutex
>
>
> Anything else is buggy.
>
> So, either your condvar use is wrong or you don't really want a condvar
> in the first place. I haven't checked the code.
>
A flag is needed in the vcpu structure that indicates whether the vcpu
spun up or not. This is what the while () condition should be. This is
needed as the thread may spin up before it gets to the
pthread_cond_wait() in which case the signal happens when noone is
waiting on it. The other reason a while () is usually needed is that
cond_signal may not wake up the right thread so it's necessary to check
whether you really have something to do. Not really a problem here but
the former race is.
A condvar is definitely the right thing to use here.
Regards,
Anthony Liguori
> - --
> ➧ Ulrich Drepper ➧ Red Hat, Inc. ➧ 444 Castro St ➧ Mountain View, CA ❖
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