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

[Peter Zijlstra]

On Mon, Dec 10, 2018 at 12:32:21AM +0200, Igor Stoppa wrote:
> 
> 
> On 06/12/2018 11:44, Peter Zijlstra wrote:
> > 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.
> 
> Ok, so the 3rd case (WR_RCU_ASSIGN_PTR) could be handled outside of this
> function.
> But, since now memcpy() will be replaced by copy_to_user(), can I assume
> that also copy_to_user() will be atomic, if the destination is properly
> aligned? On x86_64 it seems yes, however it's not clear to me if this is the
> outcome of an optimization or if I can expect it to be always true.

This would be a new contraint; one that needs to be documented and
verified by the various arch maintainers as they enable this feature on
their platform.