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

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Date:   Tue, 9 Feb 2021 14:31:16 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Segher Boessenkool' <segher@...nel.crashing.org>,
        Nicholas Piggin <npiggin@...il.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Paul Mackerras" <paulus@...ba.org>,
        "msuchanek@...e.de" <msuchanek@...e.de>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>
Subject: RE: [PATCH v5 20/22] powerpc/syscall: Avoid storing 'current' in
 another pointer

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?

	David

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