The semaphore currently has two admission paths: the
obtain_permit()/with_permit() methods which admits permits on user
request (the front door) and the maybe_admit_waiters() which admits
permits based on internal events like memory resource being returned
(the back door). The two paths used their own admission conditions
and naturally this means that they diverged in time. Notably,
maybe_admit_waiters() did not look at inactive readers assuming that if
there are waiters there cannot be inactive readers. This is not true
however since we merged the execution-stage into the semaphore. Waiters
can queue up even when there are inactive reads and thus
maybe_admit_waiters() has to consider evicting some of them to see if
this would allow for admitting new reads.
To avoid such divergence in the future, the admission logic was moved
into a new method can_admit_read() which is now shared between the two
method families. This method now checks for the possibility of evicting
inactive readers as well.
The admission logic was tuned slightly to only consider evicting
inactive readers if there is a real possibility that this will result
in admissions: notably, before this patch, resource availability was
checked before stalls were (used permits == blocked permits), so we
could evict readers even if this couldn't help.
Because now eviction can be started from maybe_admit_waiters(), which is
also downstream from eviction, we added a flag to avoid recursive
evict -> maybe admit -> evict ... loops.
Fixes: #11770Closes#11784
(cherry picked from commit 7fbad8de87)
8ffdb8546b