K 10
svn:author
V 7
scottph
K 8
svn:date
V 27
2020-08-26T02:04:04.260939Z
K 7
svn:log
V 750
arm64: Make local stores observable before sending IPIs

Add a synchronizing instruction to flush and wait until the local
CPU's writes are observable to other CPUs before sending IPIs.

This fixes an issue where recipient CPUs doing a rendezvous could
enter the rendezvous handling code before the initiator's writes
to the smp_rv_* variables were visible. This manifested as a
system hang, where a single CPU's increment of smp_rv_waiters[0]
actually happened "before" the initiator's zeroing of that field,
so all CPUs were stuck with the field appearing to be at
ncpus - 1.

Reviewed by:	andrew, markj
Approved by:	scottl (implicit)
MFC after:	1 week
Sponsored by:	Ampere Computing, Inc.
Differential Revision:	https://reviews.freebsd.org/D25798

END