Hello Steven, On Mon, Sep 21, 2020 at 7:17 PM Eads, Gage <gage.e...@intel.com> wrote: > > -----Original Message----- > > From: Steven Lariau <steven.lar...@arm.com> > > Sent: Friday, September 11, 2020 10:30 AM > > To: Eads, Gage <gage.e...@intel.com>; Olivier Matz <olivier.m...@6wind.com> > > Cc: dev@dpdk.org; n...@arm.com; dharmik.thak...@arm.com; Steven Lariau > > <steven.lar...@arm.com> > > Subject: [PATCH 5/5] lib/stack: remove pop cas release ordering > > > > Replace the store-release by relaxed for the CAS success at the end of > > pop. Release isn't needed, because there is not write to data that need > > to be synchronized. > > The only preceding write is when the length is decreased, but the length > > CAS loop already ensures the right synchronization. > > The situation to avoid is when a thread sees the old length but the new > > list, that doesn't have enough items for pop to success. > > But the CAS success on length before the pop loop ensures any core reads > > and updates the latest length, preventing this situation. > > > > The store-release is also used to make sure that the items are read > > before the head is updated, in order to prevent a core in pop to read an > > incorrect value because another core rewrites it with push. > > But this isn't needed, because items are read only when removed from the > > used list. Right after this, they are pushed to the free list, and the > > store-release in push makes sure the items are read before they are > > visible in the free list. > > Please add a comment to this effect above the compare-exchange call. Depending > on this caller behavior for correctness is a little risky, but since this > header > is private to the library I think it's ok as long as it's well-documented. >
Can you prepare a v2 for this comment? Thanks. -- David Marchand
