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

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Date:   Wed, 10 Feb 2021 09:45:56 +0100
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Nicholas Piggin <npiggin@...il.com>,
        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



Le 10/02/2021 à 03:00, Nicholas Piggin a écrit :
> 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,

I'll post v6 of it as a follow-up of yesterday's two remaining follow-up patches.

Christophe