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SubjectRe: [PATCHv3 1/1] locking/qspinlock/x86: Avoid test-and-set when PV_DEDICATED is set
On Fri, Nov 10, 2017 at 10:07:56AM +0800, Wanpeng Li wrote:

> >> Also, you should not put cpumask_t on stack, that's 'broken'.
>
> Thanks pointing out this. I found a useful comments in arch/x86/kernel/irq.c:
>
> /* These two declarations are only used in check_irq_vectors_for_cpu_disable()
> * below, which is protected by stop_machine(). Putting them on the stack
> * results in a stack frame overflow. Dynamically allocating could result in a
> * failure so declare these two cpumasks as global.
> */
> static struct cpumask affinity_new, online_new;

That code no longer exists.. Also not entirely sure how it would be
helpful.

What you probably want to do is have a per-cpu cpumask, since
flush_tlb_others() is called with preemption disabled. But you probably
don't want an unconditionally allocated one, since most kernels will not
in fact be PV.

So you'll want something like:

static DEFINE_PER_CPU(cpumask_var_t, __pv_tlb_mask);

And then you need something like:

for_each_possible_cpu(cpu) {
zalloc_cpumask_var_node(per_cpu_ptr(&__pb_tlb_mask, cpu),
GFP_KERNEL, cpu_to_node(cpu));
}

before you set the pv-op or so.

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