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SubjectRe: Why is get_current() not const function?
On Mon, Mar 17, 2003 at 06:26:05PM +0100, Andi Kleen wrote:
> torvalds@transmeta.com (Linus Torvalds) writes:
>
> >> and why on x86-64
> >>the movq %%gs:0, %0 inline asm is volatile with "memory" clobber?
> >
> > Can't help you on that one, but it looks like it uses various helper
> > functions for doing the x86-64 per-processor data structures, and I bet
> > those helper functions are shared by _other_ users who definitely want
> > to have their data properly re-read. Ie "current()" may be constant in
> > process context, but that sure isn't true about a lot of other things in
> > the per-processor data structures.
>
> Yes, that's the big issue. const current requires non volatile read_pda()
> and making read_pda non volatile breaks lots of code currently and probably
> needs an audit over all users.

Well, that's one that is not particularly hard to fix.
Either a new set of pda access macros without volatile and memory clobber
can be written, or just get_current can use its own asm, ie.
static inline struct task_struct *get_current(void)
{
struct task_struct *t;
asm ("movq %%gs:%c1,%0" : "=r" (t) : "i"(pda_offset(pcurrent)));
return t;
}

Jakub
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