[PATCH v6 4/4] kdb: Don't back trace on a cpu that didn't round up

[Douglas Anderson <dianders@chromium.org>]

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@chromium.org>
---

Changes in v6: None
Changes in v5: None
Changes in v4:
- Also clear out .debuggerinfo.
- Also clear out .debuggerinfo and .task for the master.
- Remove clearing out in kdb_stub for offline CPUs; it's now redundant.

Changes in v3:
- Don't back trace on a cpu that didn't round up new for v3.

Changes in v2: None

 kernel/debug/debug_core.c       |  4 ++++
 kernel/debug/kdb/kdb_bt.c       | 11 ++++++++++-
 kernel/debug/kdb/kdb_debugger.c |  7 -------
 3 files changed, 14 insertions(+), 8 deletions(-)

diff --git a/kernel/debug/debug_core.c b/kernel/debug/debug_core.c
index 1fb8b239e567..5cc608de6883 100644
--- a/kernel/debug/debug_core.c
+++ b/kernel/debug/debug_core.c
@@ -592,6 +592,8 @@ static int kgdb_cpu_enter(struct kgdb_state *ks, struct pt_regs *regs,
 				arch_kgdb_ops.correct_hw_break();
 			if (trace_on)
 				tracing_on();
+			kgdb_info[cpu].debuggerinfo = NULL;
+			kgdb_info[cpu].task = NULL;
 			kgdb_info[cpu].exception_state &=
 				~(DCPU_WANT_MASTER | DCPU_IS_SLAVE);
 			kgdb_info[cpu].enter_kgdb--;
@@ -724,6 +726,8 @@ static int kgdb_cpu_enter(struct kgdb_state *ks, struct pt_regs *regs,
 	if (trace_on)
 		tracing_on();
 
+	kgdb_info[cpu].debuggerinfo = NULL;
+	kgdb_info[cpu].task = NULL;
 	kgdb_info[cpu].exception_state &=
 		~(DCPU_WANT_MASTER | DCPU_IS_SLAVE);
 	kgdb_info[cpu].enter_kgdb--;
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();
 		}
diff --git a/kernel/debug/kdb/kdb_debugger.c b/kernel/debug/kdb/kdb_debugger.c
index 15e1a7af5dd0..53a0df6e4d92 100644
--- a/kernel/debug/kdb/kdb_debugger.c
+++ b/kernel/debug/kdb/kdb_debugger.c
@@ -118,13 +118,6 @@ int kdb_stub(struct kgdb_state *ks)
 	kdb_bp_remove();
 	KDB_STATE_CLEAR(DOING_SS);
 	KDB_STATE_SET(PAGER);
-	/* zero out any offline cpu data */
-	for_each_present_cpu(i) {
-		if (!cpu_online(i)) {
-			kgdb_info[i].debuggerinfo = NULL;
-			kgdb_info[i].task = NULL;
-		}
-	}
 	if (ks->err_code == DIE_OOPS || reason == KDB_REASON_OOPS) {
 		ks->pass_exception = 1;
 		KDB_FLAG_SET(CATASTROPHIC);
-- 
2.20.0.rc0.387.gc7a69e6b6c-goog