Re: [PATCH 06/14] migration: Use atomic ops properly for page accountings

[Peter Xu <peterx@redhat.com>]

On Tue, Oct 04, 2022 at 05:59:36PM +0100, Dr. David Alan Gilbert wrote:
> * Peter Xu (peterx@redhat.com) wrote:
> > To prepare for thread-safety on page accountings, at least below counters
> > need to be accessed only atomically, they are:
> > 
> >         ram_counters.transferred
> >         ram_counters.duplicate
> >         ram_counters.normal
> >         ram_counters.postcopy_bytes
> > 
> > There are a lot of other counters but they won't be accessed outside
> > migration thread, then they're still safe to be accessed without atomic
> > ops.
> > 
> > Signed-off-by: Peter Xu <peterx@redhat.com>
> 
> I think this is OK; I'm not sure whether the memset 0's of ram_counters
> technically need changing.

IMHO they're fine - what we need there should be thing like WRITE_ONCE()
just to make sure no register caches (actually atomic_write() is normally
implemented with WRITE_ONCE afaik).  But I think that's already guaranteed
by memset() as the function call does, so we should be 100% safe.

> I'd love to put a comment somewhere saying these fields need to be
> atomically read, but their qapi defined so I don't think we can.

How about I add a comment above ram_counters declarations in ram.c?

> 
> Finally, we probably need to check these are happy on 32 bit builds,
> sometimes it's a bit funny with atomic adds.

Yeah.. I hope using qatomic_*() APIs can help me avoid any issues.  Or
anything concerning?  I'd be happy to test on specific things if there are.

> 
> 
> Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

Thanks!

-- 
Peter Xu