> -----Original Message-----
> From: Steven Lariau <[email protected]>
> Sent: Friday, September 11, 2020 10:30 AM
> To: Eads, Gage <[email protected]>; Olivier Matz <[email protected]>
> Cc: [email protected]; [email protected]; [email protected]; Steven Lariau
> <[email protected]>
> 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.
Thanks,
Gage
