[Bug 271826] FreeBSD is disastrously slow on a PowerMac G5, freezing at every command

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=271826

--- Comment #31 from Justin Hibbits <jhibbits@FreeBSD.org> ---
Comment on attachment 251202
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=251202
tbsync patch 1

All the atomic_set_*()s should really be atomic_store_*(), `atomic_set` is a
bit-set (OR operation).  It's incorrect in the existing code, too.  It really
doesn't make a significant difference, since the end result is the same
(cpu_done should be 0 to start with, so setting nothing doesn't change that,
tb_ready should be 0 to start with, so setting 1 bit does the same thing),
except in the last instance to try to clear tb_ready.

In the tight loop checking if cpu_done is high enough, you don't need the acq
barrier in there, because the thread fence occupying the loop does the same
thing, and acq places the barrier after the operation, so in the same place
anyway.

Glad to see this patch overall works for you, though!  I'm really surprised
those barriers are needed, since they're not needed in the mpc85xx case, which
this is derived from.

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