schedinit_ap() sets up an AP for a later call to sched_throw(NULL).
Currently, ULE sets up some pcpu bits and fixes the idlethread lock with
a call to sched_throw(NULL); this results in a window where curthread is
setup in platforms' init_secondary(), but it has the wrong td_lock.
Typical platform AP startup procedure looks something like:
- Setup curthread
- ... other stuff, including cpu_initclocks_ap()
- Signal smp_started
- sched_throw(NULL) to enter the scheduler
cpu_initclocks_ap() may have callouts to process (e.g., nvme) and
attempt to sched_add() for this AP, but this attempt fails because
of the noted violated assumption leading to locking heartburn in
sched_setpreempt().
Interrupts are still disabled until cpu_throw() so we're not really at
risk of being preempted -- just let the scheduler in on it a little
earlier as part of setting up curthread.
Reviewed by: alfredo, kib, markj
Triage help from: andrew, markj
Smoke-tested by: alfredo (ppc), kevans (arm64, x86), mhorne (arm)
Differential Revision: https://reviews.freebsd.org/D32797
589aed00e3