[PATCH] cpu/hotplug: Provide dynamic range for prepare stage

[Thomas Gleixner]

Mathieu reported that the LTTNG modules are broken as of 4.10-rc1 due to
the removal of the cpu hotplug notifiers.

Usually I don't care much about out of tree modules, but LTTNG is widely
used in distros. There are two ways to solve that:

1) Reserve a hotplug state for LTTNG

2) Add a dynamic range for the prepare states.

While #1 is the simplest solution, #2 is the proper one as we can convert
in tree users, which do not care about ordering, to the dynamic range as
well.

Add a dynamic range which allows LTTNG to request states in the prepare
stage.

Signed-off-by: Thomas Gleixner <t...@linutronix.de>

---

diff --git a/include/linux/cpuhotplug.h b/include/linux/cpuhotplug.h
index 20bfefbe7594..d936a0021839 100644
--- a/include/linux/cpuhotplug.h
+++ b/include/linux/cpuhotplug.h
@@ -74,6 +74,8 @@ enum cpuhp_state {
        CPUHP_ZCOMP_PREPARE,
        CPUHP_TIMERS_DEAD,
        CPUHP_MIPS_SOC_PREPARE,
+       CPUHP_BP_PREPARE_DYN,
+       CPUHP_BP_PREPARE_DYN_END                = CPUHP_BP_PREPARE_DYN + 20,
        CPUHP_BRINGUP_CPU,
        CPUHP_AP_IDLE_DEAD,
        CPUHP_AP_OFFLINE,
diff --git a/kernel/cpu.c b/kernel/cpu.c
index f75c4d031eeb..c47506357519 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -1302,10 +1302,24 @@ static int cpuhp_cb_check(enum cpuhp_state state)
  */
 static int cpuhp_reserve_state(enum cpuhp_state state)
 {
-       enum cpuhp_state i;
+       enum cpuhp_state i, end;
+       struct cpuhp_step *step;
 
-       for (i = CPUHP_AP_ONLINE_DYN; i <= CPUHP_AP_ONLINE_DYN_END; i++) {
-               if (!cpuhp_ap_states[i].name)
+       switch (state) {
+       case CPUHP_AP_ONLINE_DYN:
+               step = cpuhp_ap_states + CPUHP_AP_ONLINE_DYN;
+               end = CPUHP_AP_ONLINE_DYN_END;
+               break;
+       case CPUHP_BP_PREPARE_DYN:
+               step = cpuhp_bp_states + CPUHP_BP_PREPARE_DYN;
+               end = CPUHP_BP_PREPARE_DYN_END;
+               break;
+       default:
+               return -EINVAL;
+       }
+
+       for (i = state; i <= end; i++, step++) {
+               if (!step->name)
                        return i;
        }
        WARN(1, "No more dynamic states available for CPU hotplug\n");
@@ -1323,7 +1337,7 @@ static int cpuhp_store_callbacks(enum cpuhp_state state, 
const char *name,
 
        mutex_lock(&cpuhp_state_mutex);
 
-       if (state == CPUHP_AP_ONLINE_DYN) {
+       if (state == CPUHP_AP_ONLINE_DYN || state == CPUHP_BP_PREPARE_DYN) {
                ret = cpuhp_reserve_state(state);
                if (ret < 0)
                        goto out;