Merge branch 'smp-hotplug-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull CPU hotplug fix from Thomas Gleixner:
 "A single fix to handle the removal of the first dynamic CPU hotplug
  state correctly"

* 'smp-hotplug-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  smp/hotplug: Handle removal correctly in cpuhp_store_callbacks()

1 files changed, 11 insertions, 1 deletions

diff --git a/kernel/cpu.c b/kernel/cpu.c
index bfbd649ccdc8..acf5308fad51 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -1253,7 +1253,17 @@ 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;