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

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

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))

(asm-i386/atomic.h)

And that's really far from beeing the 2 only cases where the kernel _relies_
on a write of a simple type like int or long to an aligned location to be
atomic. Almost all drivers manipulating DMA descriptors do that, jiffies
is a good example too afaik, and more and more and more ... so if the
compiler is breaking that up, I think the set_pte race is the least of
our problems :)

Ben.