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

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Date:   Wed, 3 Oct 2018 17:07:36 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Christophe LEROY <christophe.leroy@....fr>
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

On Wed, 3 Oct 2018 08:45:25 +0200
Christophe LEROY <christophe.leroy@....fr> wrote:

> 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 ?

In patch 3 we can, right? That's what I mean by moving from
thread_info * to void * in patch 3 rather than 4.

But if you prefer not to, it's fine. Maybe it keeps patch 3
a little smaller.

Thanks,
Nick