cpu/hotplug: Provide dynamic range for prepare stage

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.

Reported-and-tested-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sebastian Sewior <bigeasy@linutronix.de>
Cc: Steven Rostedt <rostedt@goodmis.org>
Link: http://lkml.kernel.org/r/alpine.DEB.2.20.1701101353010.3401@nanos
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

2 files changed, 20 insertions, 4 deletions

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;