Re: [PATCH] powerpc: Ensure gcc doesn't move around cache flushing in __patch_instruction

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Thu, 2018-05-17 at 14:23 -0500, Segher Boessenkool wrote:
> Hi!
> 
> On Thu, May 17, 2018 at 01:06:10PM +1000, Benjamin Herrenschmidt wrote:
> > The current asm statement in __patch_instruction() for the cache flushes
> > doesn't have a "volatile" statement and no memory clobber. That means
> > gcc can potentially move it around (or move the store done by put_user
> > past the flush).
> 
> volatile is completely superfluous here, except maybe as documentation:
> any asm without outputs is always volatile.

I wasn't aware of that. I was drilled early on to always stick volatile
in my asm statements if they have any form of side effect :-)

> (And the memory clobber does not prevent the compiler from moving the
> asm around, or duplicating it, etc., and neither does the volatile).

It prevents load/stores from moving around doesn't it ? I wanted to
make sure the store of the instruction doesn't move in/pass the asm. If
you say that's not needed then ignore the patch.

Cheers,
Ben.
 
> 
> Segher