[GIT pull] cpu hotplug updates for 4.14

[Thomas Gleixner]

Linus,

please pull the latest smp-hotplug-for-linus git tree from:

   git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git 
smp-hotplug-for-linus

A single fix to handle the removal of the first dynamic CPU hotplug state
correctly.

Thanks,

        tglx

------------------>
Ethan Barnes (1):
      smp/hotplug: Handle removal correctly in cpuhp_store_callbacks()


 kernel/cpu.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/kernel/cpu.c b/kernel/cpu.c
index eee033134262..a88c29ab09be 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -1252,7 +1252,17 @@ static int cpuhp_store_callbacks(enum cpuhp_state state, 
const char *name,
        struct cpuhp_step *sp;
        int ret = 0;
 
-       if (state == CPUHP_AP_ONLINE_DYN || state == CPUHP_BP_PREPARE_DYN) {
+       /*
+        * If name is NULL, then the state gets removed.
+        *
+        * CPUHP_AP_ONLINE_DYN and CPUHP_BP_PREPARE_DYN are handed out on
+        * the first allocation from these dynamic ranges, so the removal
+        * would trigger a new allocation and clear the wrong (already
+        * empty) state, leaving the callbacks of the to be cleared state
+        * dangling, which causes wreckage on the next hotplug operation.
+        */
+       if (name && (state == CPUHP_AP_ONLINE_DYN ||
+                    state == CPUHP_BP_PREPARE_DYN)) {
                ret = cpuhp_reserve_state(state);
                if (ret < 0)
                        return ret;