risc-v: Fix order of IPI enablement vs RCU startup

Conor reports that risc-v tries to enable IPIs before telling the
core code to enable RCU. With the introduction of the mapple tree
as a backing store for the irq descriptors, this results in
a very shouty boot sequence, as RCU is legitimately upset.

Restore some sanity by moving the risc_ipi_enable() call after
notify_cpu_starting(), which explicitly enables RCU on the calling
CPU.

Fixes: 832f15f ("RISC-V: Treat IPIs as normal Linux IRQs")
Reported-by: Conor Dooley <[email protected]>
Signed-off-by: Marc Zyngier <[email protected]>
Link: https://lore.kernel.org/r/20230703-dupe-frying-79ae2ccf94eb@spud
Cc: Anup Patel <[email protected]>
Cc: Palmer Dabbelt <[email protected]>
Cc: Linus Torvalds <[email protected]>
Tested-by: Conor Dooley <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Palmer Dabbelt <[email protected]>

Author: Marc Zyngier

arch/riscv/kernel/smpboot.c

@@ -238,10 +238,11 @@ asmlinkage __visible void smp_callin(void)
 	mmgrab(mm);
 	current->active_mm = mm;
 
-	riscv_ipi_enable();
-
 	store_cpu_topology(curr_cpuid);
 	notify_cpu_starting(curr_cpuid);
+
+	riscv_ipi_enable();
+
 	numa_add_cpu(curr_cpuid);
 	set_cpu_online(curr_cpuid, 1);
 	probe_vendor_features(curr_cpuid);