linux-kernel - Re: [RFC PATCH v3 4/7] powerpc: regain entire stack space

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Date:   Wed, 3 Oct 2018 08:45:25 +0200
From:   Christophe LEROY <christophe.leroy@....fr>
To:     Nicholas Piggin <npiggin@...il.com>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        aneesh.kumar@...ux.vnet.ibm.com, linux-kernel@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org
Subject: Re: [RFC PATCH v3 4/7] powerpc: regain entire stack space



Le 03/10/2018 à 08:30, Nicholas Piggin a écrit :
> On Wed, 3 Oct 2018 07:52:59 +0200
> Christophe LEROY <christophe.leroy@....fr> wrote:
> 
>> Le 03/10/2018 à 07:34, Nicholas Piggin a écrit :
>>> On Mon,  1 Oct 2018 12:30:25 +0000 (UTC)
>>> Christophe Leroy <christophe.leroy@....fr> wrote:
>>>    
>>>> thread_info is not anymore in the stack, so the entire stack
>>>> can now be used.
>>>
>>> Nice.
>>>    
>>>>
>>>> In the meantime, all pointers to the stacks are not anymore
>>>> pointers to thread_info so this patch changes them to void*
>>>
>>> Wasn't this previously effectively already the case with patch
>>> 3/7? You had thread_info sized space left there, but it was not
>>> used or initialized right? Does it make sense to move this part
>>> of it to the previous patch?
>>
>> Not really.
>>
>> In 3/7 I changed the prototypes of two functions that really used the
>> pointer as a task pointer only.

I meant 2/7 here sorry.

>>
>> Here it change things that before 4/7 were really used as both stack
>> pointers and thread_info pointers.

And here I meant 3/7

> 
> What uses it as a thread_info pointer? It seems more like a stack
> with some amount of unused space in it but that's all.

Before 3/7, we have

void do_softirq_own_stack(void)
{
	struct thread_info *curtp, *irqtp;

	curtp = current_thread_info();
	irqtp = softirq_ctx[smp_processor_id()];
	irqtp->task = curtp->task;
	irqtp->flags = 0;
	call_do_softirq(irqtp);
	irqtp->task = NULL;

	/* Set any flag that may have been set on the
	 * alternate stack
	 */
	if (irqtp->flags)
		set_bits(irqtp->flags, &curtp->flags);
}

After 3/7, we have

  void do_softirq_own_stack(void)
  {
	struct thread_info *irqtp;

  	irqtp = softirq_ctx[smp_processor_id()];
  	call_do_softirq(irqtp);
  }


So now only we can change irqtp to void* can't we ?

> 
> That said I don't care to nitpick too much where things go exactly
> if you like it better here that's fine.

No worry, I may have missed something, your comments are always welcome.

Thanks
Christophe

> 
> Thanks,
> Nick
>