Il 26/06/2013 10:06, Srivatsa S. Bhat ha scritto: > On 06/26/2013 01:16 PM, Paolo Bonzini wrote: >> Il 25/06/2013 22:30, Srivatsa S. Bhat ha scritto: >>> - cpu = get_cpu(); >>> + cpu = get_online_cpus_atomic(); >>> vmx_vcpu_load(&vmx->vcpu, cpu); >>> vmx->vcpu.cpu = cpu; >>> err = vmx_vcpu_setup(vmx); >>> vmx_vcpu_put(&vmx->vcpu); >>> - put_cpu(); >>> + put_online_cpus_atomic(); >> >> The new API has a weird name. Why are you adding new functions instead >> of just modifying get/put_cpu? >> > > Because the purpose of those two functions are distinctly different > from each other. > > get/put_cpu() is used to disable preemption on the local CPU. (Which > also disables offlining the local CPU during that critical section). Ok, then I understood correctly... and I acked the other KVM patch. However, keeping the code on the local CPU is exactly the point of this particular use of get_cpu()/put_cpu(). Why does it need to synchronize with offlining of other CPUs? Paolo > What this patchset deals with is synchronizing with offline of *any* > CPU. Typically, we use get_online_cpus()/put_online_cpus() for that > purpose. But they can't be used in atomic context, because they take > mutex locks and hence can sleep. > > So the code that executes in atomic context and which wants to prevent > *any* CPU from going offline, used to disable preemption around its > critical section. Disabling preemption prevents stop_machine(), and > CPU offline (of *any* CPU) was done via stop_machine(). So disabling > preemption disabled any CPU from going offline, as a *side-effect*. > > And this patchset prepares the ground for getting rid of stop_machine() > in the CPU offline path. Which means, disabling preemption only prevents > the *local* CPU from going offline. So if code in atomic context wants > to prevent any CPU from going offline, we need a new set of APIs, like > get/put_online_cpus(), but which can be invoked from atomic context. > That's why I named it as get/put_online_cpus_atomic(). > > One of the key points here is that we want to preserve get/put_cpu() > as it is, since its purpose is different - disable preemption and > offline of the local CPU. There is no reason to change that API, its > useful as it is. -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
