Pull up following revision(s) (requested by bouyer in ticket #326):
sys/arch/arm/vfp/vfp_init.c: revision 1.54-1.55
sys/kern/subr_pcu.c: revision 1.21
PR port-arm/52603:
There is a race here, as seen on arm with FPU:
LWP L is running but not on CPU, has its FPU state on CPU2 which
has not been released yet, so fpexc still has VFP_FPEXC_EN set in the PCB copy.
LWP L is scheduled on CPU1, CPU1 calls cpu_switchto() for L in mi_switch().
cpu_switchto() will set VFP_FPEXC_EN in the FPU's fpexc register per the
PCB fpexc copy.
Before CPU1 calls pcu_switchpoint() for L, CPU2 calls
pcu_do_op(PCU_CMD_SAVE | PCU_CMD_RELEASE) for L because it still holds its
FPU state and wants to load another lwp. This cause VFP_FPEXC_EN to
be cleared in the PCB copy, but not in CPU1's register. L's l_pcu_cpu is
set to NULL.
When CPU1 calls pcu_switchpoint() for L it see l_pcu_cpu is NULL, and doesn't
call the release callback.
Now CPU1 has its FPU enabled but with the wrong FPU state.
Fix by releasing the PCU even if l_pcu_cpu is NULL.
--
In the REENABLE case, make sur the fpexc copy in the pcb also has
VFP_FPEXC_EN set. Otherwise we could trap on every context switch even if
the CPU already has the VFP state.
--
We KASSERT((fregs->vfp_fpexc & VFP_FPEXC_EN) == 0) just before, so
enabled is always false. remove.

(snj)