RE: [PATCH v5 20/22] powerpc/syscall: Avoid storing 'current' in another pointer

[David Laight <David.Laight@ACULAB.COM>]

From: Christophe Leroy <christophe.leroy@csgroup.eu>
> Sent: 09 February 2021 17:04
> 
> 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.
> 
> 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.

Does marking current_thread_info() 'pure' (I think that the right one)
work - so that gcc knows its result doesn't depend on external data
and that it doesn't change external data.

Although I'm not 100% how well those attributes actually work.

> Nick, I don't understand the reason why you need that 'ti_flagsp' local var.

Probably to save typing.

I sometimes reload locals after function calls.

	David

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