linux-kernel - Re: [PATCH v5 20/22] powerpc/syscall: Avoid storing 'current' in another pointer

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Message-Id: <1612922312.mnpowzgd0r.astroid@bobo.none>
Date:   Wed, 10 Feb 2021 12:00:20 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Christophe Leroy <christophe.leroy@...roup.eu>,
        David Laight <David.Laight@...LAB.COM>,
        'Segher Boessenkool' <segher@...nel.crashing.org>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        "msuchanek@...e.de" <msuchanek@...e.de>,
        Paul Mackerras <paulus@...ba.org>
Subject: Re: [PATCH v5 20/22] powerpc/syscall: Avoid storing 'current' in
 another pointer

Excerpts from Christophe Leroy's message of February 10, 2021 3:03 am:
> 
> 
> Le 09/02/2021 à 15:31, David Laight a écrit :
>> From: Segher Boessenkool
>>> Sent: 09 February 2021 13:51
>>>
>>> On Tue, Feb 09, 2021 at 12:36:20PM +1000, Nicholas Piggin wrote:
>>>> What if you did this?
>>>
>>>> +static inline struct task_struct *get_current(void)
>>>> +{
>>>> +	register struct task_struct *task asm ("r2");
>>>> +
>>>> +	return task;
>>>> +}
>>>
>>> Local register asm variables are *only* guaranteed to live in that
>>> register as operands to an asm.  See
>>>    https://gcc.gnu.org/onlinedocs/gcc/Local-Register-Variables.html#Local-Register-Variables
>>> ("The only supported use" etc.)
>>>
>>> You can do something like
>>>
>>> static inline struct task_struct *get_current(void)
>>> {
>>> 	register struct task_struct *task asm ("r2");
>>>
>>> 	asm("" : "+r"(task));
>>>
>>> 	return task;
>>> }
>>>
>>> which makes sure that "task" actually is in r2 at the point of that asm.
>> 
>> If "r2" always contains current (and is never assigned by the compiler)
>> why not use a global register variable for it?
>> 
> 
> 
> The change proposed by Nick doesn't solve the issue.

It seemed to change code generation in a simple test case, oh well.

> 
> The problem is that at the begining of the function we have:
> 
> 	unsigned long *ti_flagsp = &current_thread_info()->flags;
> 
> When the function uses ti_flagsp for the first time, it does use 112(r2)
> 
> Then the function calls some other functions.
> 
> Most likely because the function could update 'current', GCC copies r2 into r30, so that if r2 get 
> changed by the called function, ti_flagsp is still based on the previous value of current.
> 
> Allthough we know r2 wont change, GCC doesn't know it. And in order to save r2 into r30, it needs to 
> save r30 in the stack.
> 
> 
> By using &current_thread_info()->flags directly instead of this intermediaite ti_flagsp pointer, GCC 
> uses r2 instead instead of doing a copy.
> 
> 
> Nick, I don't understand the reason why you need that 'ti_flagsp' local var.

Just to save typing, I don't mind your patch I was just wondering if 
current could be improved in general.

Thanks,
Nick