Re: [PATCH 2/6] __wr_after_init: write rare for static allocation

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Dec 05, 2018 at 03:13:56PM -0800, Andy Lutomirski wrote:

> > +       if (op == WR_MEMCPY)
> > +               memcpy((void *)wr_poking_addr, (void *)src, len);
> > +       else if (op == WR_MEMSET)
> > +               memset((u8 *)wr_poking_addr, (u8)src, len);
> > +       else if (op == WR_RCU_ASSIGN_PTR)
> > +               /* generic version of rcu_assign_pointer */
> > +               smp_store_release((void **)wr_poking_addr,
> > +                                 RCU_INITIALIZER((void **)src));
> > +       kasan_enable_current();
> 
> Hmm.  I suspect this will explode quite badly on sane architectures
> like s390.  (In my book, despite how weird s390 is, it has a vastly
> nicer model of "user" memory than any other architecture I know
> of...).  I think you should use copy_to_user(), etc, instead.  I'm not
> entirely sure what the best smp_store_release() replacement is.
> Making this change may also mean you can get rid of the
> kasan_disable_current().

If you make the MEMCPY one guarantee single-copy atomicity for native
words then you're basically done.

smp_store_release() can be implemented with:

	smp_mb();
	WRITE_ONCE();

So if we make MEMCPY provide the WRITE_ONCE(), all we need is that
barrier, which we can easily place at the call site and not overly
complicate our interface with this.

Because performance is down the drain already, an additional full
memory barrier is peanuts here (and in fact already implied by the x86
CR3 munging).