[PATCH v3 4/4] kdb: Don't back trace on a cpu that didn't round up

[Douglas Anderson]

If you have a CPU that fails to round up and then run 'btc' you'll end
up crashing in kdb becaue we dereferenced NULL.  Let's add a check.
It's wise to also set the task to NULL when leaving the debugger so
that if we fail to round up on a later entry into the debugger we
won't backtrace a stale task.

Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
---

Changes in v3:
- New for v3.

Changes in v2: None

 kernel/debug/debug_core.c |  1 +
 kernel/debug/kdb/kdb_bt.c | 11 ++++++++++-
 2 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/kernel/debug/debug_core.c b/kernel/debug/debug_core.c
index 324cba8917f1..08851077c20a 100644
--- a/kernel/debug/debug_core.c
+++ b/kernel/debug/debug_core.c
@@ -587,6 +587,7 @@ static int kgdb_cpu_enter(struct kgdb_state *ks, struct pt_regs *regs,
 			kgdb_info[cpu].exception_state &=
 				~(DCPU_WANT_MASTER | DCPU_IS_SLAVE);
 			kgdb_info[cpu].enter_kgdb--;
+			kgdb_info[cpu].task = NULL;
 			smp_mb__before_atomic();
 			atomic_dec(&slaves_in_kgdb);
 			dbg_touch_watchdogs();
diff --git a/kernel/debug/kdb/kdb_bt.c b/kernel/debug/kdb/kdb_bt.c
index 7921ae4fca8d..7e2379aa0a1e 100644
--- a/kernel/debug/kdb/kdb_bt.c
+++ b/kernel/debug/kdb/kdb_bt.c
@@ -186,7 +186,16 @@ kdb_bt(int argc, const char **argv)
 		kdb_printf("btc: cpu status: ");
 		kdb_parse("cpu\n");
 		for_each_online_cpu(cpu) {
-			sprintf(buf, "btt 0x%px\n", KDB_TSK(cpu));
+			void *kdb_tsk = KDB_TSK(cpu);
+
+			/* If a CPU failed to round up we could be here */
+			if (!kdb_tsk) {
+				kdb_printf("WARNING: no task for cpu %ld\n",
+					   cpu);
+				continue;
+			}
+
+			sprintf(buf, "btt 0x%px\n", kdb_tsk);
 			kdb_parse(buf);
 			touch_nmi_watchdog();
 		}
-- 
2.19.1.930.g4563a0d9d0-goog