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). 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. Regards, Srivatsa S. Bhat -- 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
