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

[Igor Stoppa <igor.stoppa@gmail.com>]

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.


--
igor