Re: [Xen-devel] [PATCH 3/3] x86: introduce and use scratch CPU mask

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 15/12/16 14:59, Jan Beulich wrote:
>>>> On 15.12.16 at 15:15, <andrew.cooper3@xxxxxxxxxx> wrote:
>> On 09/12/16 08:59, Jan Beulich wrote:
>>>>>> On 08.12.16 at 18:51, <andrew.cooper3@xxxxxxxxxx> wrote:
>>>> On 08/12/16 16:02, Jan Beulich wrote:
>>>>> variable. With an IRQ happening at the deepest point of the stack, and
>>>>> with send_guest_pirq() being called from there (leading to vcpu_kick()
>>>>> -> ... -> csched_vcpu_wake() -> __runq_tickle() ->
>>>>> cpumask_raise_softirq(), the last two of which also have CPU mask
>>>>> variables on their stacks), this has been observed to cause a stack
>>>> "stacks), has been".
>>> Hmm, that looks strange to me: Wouldn't the dropping of "this"
>>> also requite the "With" at the start of the sentence to be dropped
>>> (and isn't the sentence okay with both left in place)?
>> The use of "this" requires a substantial backtrack through the text to
>> evaluate what it refers to, which as you point, I didn't manage to do
>> successfully. 
>>
>> As is evident, I had a very hard time trying to parse the sentence.
>>
>> It would be clearer to read if you took out both the "With" and "this".
> I've re-ordered it some more:
>
> __get_page_type(), so far using an on-stack CPU mask variable, is
> involved in recursion when e.g. pinning page tables. This means there
> may be up to five instances of the function active at a time, implying
> five instances of the (up to 512 bytes large) CPU mask variable. An IRQ
> happening at the deepest point of the stack has been observed to cause
> a stack overflow with a 4095-pCPU build, when the IRQ handling results
> in send_guest_pirq() being called (leading to vcpu_kick() -> ... ->
> csched_vcpu_wake() -> __runq_tickle() -> cpumask_raise_softirq(), the
> last two of which also have CPU mask variables on their stacks).
>
> Does this suit you better?

Much clearer, thanks.

~Andrew

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