On Tue 01-10-19 10:36:04, Peter Zijlstra wrote: > On Tue, Oct 01, 2019 at 09:10:08AM +0200, Jan Kara wrote: > > On Mon 30-09-19 10:57:08, John Hubbard wrote: > > > > Well, a couple of questions: > > > > > > 1. Is there *really* a memory barrier in try_get_compound_head()? Because > > > I only see a READ_ONCE compile barrier, which would mean that run time > > > reordering is still possible. > > > > try_get_compound_head() has page_cache_add_speculative() which is > > atomic_add_unless() which is guaranteed to provide ordering. > > atomic_add_unless() only provides ordering on SUCCESS, now looking at > the code in question that seems sufficient, so all should be well. Just > wanted to let you know that if atomic_add_unless() FAILs there is no > implied ordering what so ever. Yeah, right, thanks for correction! Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
