Re: [PATCH v4 1/1] powerpc: enable dynamic preemption

[Christophe Leroy (CS GROUP)]

Le 30/07/2026 à 20:40, Segher Boessenkool a écrit :
> On Thu, Jul 30, 2026 at 10:26:58AM -0700, Paul E. McKenney wrote:
> > On Thu, Jul 30, 2026 at 10:40:38PM +0530, Shrikanth Hegde wrote:
> > > Barrier are in core implementation, not in arch specific.
>
> "Compiler barrier"s are not actually a thing, it is not a barrier for
> anything.  It's just telling the compiler that at the point of the
> "barrier" all the registers should hold the values that they
> conceptually do also *actually*.
>
> (In theory the compiler can sometimes prove it can do things that do not
> even guarantee that, but in practice this holds).
>
> > But barrier() is just "__asm__ __volatile__("": : :"memory")", which
> > does not emit any instructions.  Or is this doing more machine-register
> > flushing/restoring than one might expect?
>
> (__volatile__ is redundant, any asm without outputs is always counted as
> volatile.  If this wasn't true, GCC could always delete any such asm,
> since it has no side effects at all!

There is a comment for the volatile:

/* Optimization barrier */
#ifndef barrier
/* The "volatile" is due to gcc bugs */
# define barrier() __asm__ __volatile__("": : :"memory")
#endif

> People often think "volatile" asm is some magic that prohibits the
> compiler from optimising stuff, but it is not, it has very contrained
> and very specific meaning).
>
> Nope.  The "memory" clobber says that all memory can be read and written
> in that asm, so any value that conceptually resides in memory there
> should have a stable value there.  This can trigger some save/restore
> stuff, sure, but on all Power ABIs we have actual registers for pretty
> much everything, so nothing at all is done here.
>
>
> Segher