Re: ptep_establish/establish_pte needs set_pte_atomic and all set_pte must be written in asm

[Paul Mackerras <paulus@samba.org>]

Benjamin Herrenschmidt writes:

> On Sun, 2004-09-26 at 10:46, Andrea Arcangeli wrote:
> 
> > As far as the C language is concerned that *ptep = something can be
> > implemented with 8 writes of 1 byte each (or alternatively with an
> > assembler instruction that may make the written data visible not
> > atomically to other cpus, despite it was written with a single opcode,
> > similarly to what happens if you use incl without the lock prefix). I'm
> > not saying such instruction exists in ppc64, but the compiler is
> > definitely allowed to break the above. You can blame on the compiler to
> > be inefficient, but you can't blame on the compiler for the security
> > hazard it would generate. Only the kernel would be to blame if for
> > whatever reason a gcc version would be underoptimized.
> 
> BTW, for your reading pleasure :)
> 
> #define atomic_set(v,i)		(((v)->counter) = (i))

FWIW, we also rely on several other things that are not guaranteed by
the C standard, for instance that integer arithmetic is 2's
complement, that bytes are individually addressable, and that pointers
are represented by an address that is no bigger than a long.

Paul.