[PATCH v4] panic: Fixes the panic_print NMI backtrace setting

[Guilherme G. Piccoli]

Commit 8d470a45d1a6 ("panic: add option to dump all CPUs backtraces in panic_print")
introduced a setting for the "panic_print" kernel parameter to allow
users to request a NMI backtrace on panic. Problem is that the panic_print
handling happens after the secondary CPUs are already disabled, hence
this option ended-up being kind of a no-op - kernel skips the NMI trace
in idling CPUs, which is the case of offline CPUs.

Fix it by checking the NMI backtrace bit in the panic_print prior to
the CPU disabling function.

Fixes: 8d470a45d1a6 ("panic: add option to dump all CPUs backtraces in panic_print")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Guilherme G. Piccoli <gpiccoli@xxxxxxxxxx>

---

V4:
- Sent as standalone patch, rebased against v6.2-rc7.

V2 / V3:
- New patch, there was no V1 of this one.
Link for V3: https://lore.kernel.org/lkml/20220819221731.480795-12-gpiccoli@xxxxxxxxxx/


Hi folks, thanks in advance for reviews/comments.

Notice that while at it, I got rid of the "crash_kexec_post_notifiers"
local copy in panic(). This was introduced by commit b26e27ddfd2a
("kexec: use core_param for crash_kexec_post_notifiers boot option"),
but it is not clear from comments or commit message why this local copy
is required.

My understanding is that it's a mechanism to prevent some concurrency,
in case some other CPU modify this variable while panic() is running.
I find it very unlikely, hence I removed it - but if people consider
this copy needed, I can respin this patch and keep it, even providing a
comment about that, in order to be explict about its need.

Let me know your thoughts!
Cheers,

Guilherme


 kernel/panic.c | 47 +++++++++++++++++++++++++++--------------------
 1 file changed, 27 insertions(+), 20 deletions(-)

diff --git a/kernel/panic.c b/kernel/panic.c
index 463c9295bc28..f45ee88be8a2 100644
--- a/kernel/panic.c
+++ b/kernel/panic.c
@@ -211,9 +211,6 @@ static void panic_print_sys_info(bool console_flush)
 		return;
 	}
 
-	if (panic_print & PANIC_PRINT_ALL_CPU_BT)
-		trigger_all_cpu_backtrace();
-
 	if (panic_print & PANIC_PRINT_TASK_INFO)
 		show_state();
 
@@ -243,6 +240,30 @@ void check_panic_on_warn(const char *origin)
 		      origin, limit);
 }
 
+/*
+ * Helper that triggers the NMI backtrace (if set in panic_print)
+ * and then performs the secondary CPUs shutdown - we cannot have
+ * the NMI backtrace after the CPUs are off!
+ */
+static void panic_other_cpus_shutdown(void)
+{
+	if (panic_print & PANIC_PRINT_ALL_CPU_BT)
+		trigger_all_cpu_backtrace();
+
+	/*
+	 * Note that smp_send_stop() is the usual SMP shutdown function,
+	 * which unfortunately may not be hardened to work in a panic
+	 * situation. If we want to do crash dump after notifier calls
+	 * and kmsg_dump, we will need architecture dependent extra
+	 * bits in addition to stopping other CPUs, hence we rely on
+	 * crash_smp_send_stop() for that.
+	 */
+	if (!crash_kexec_post_notifiers)
+		smp_send_stop();
+	else
+		crash_smp_send_stop();
+}
+
 /**
  *	panic - halt the system
  *	@fmt: The text string to print
@@ -258,7 +279,6 @@ void panic(const char *fmt, ...)
 	long i, i_next = 0, len;
 	int state = 0;
 	int old_cpu, this_cpu;
-	bool _crash_kexec_post_notifiers = crash_kexec_post_notifiers;
 
 	if (panic_on_warn) {
 		/*
@@ -333,23 +353,10 @@ void panic(const char *fmt, ...)
 	 *
 	 * Bypass the panic_cpu check and call __crash_kexec directly.
 	 */
-	if (!_crash_kexec_post_notifiers) {
+	if (!crash_kexec_post_notifiers)
 		__crash_kexec(NULL);
 
-		/*
-		 * Note smp_send_stop is the usual smp shutdown function, which
-		 * unfortunately means it may not be hardened to work in a
-		 * panic situation.
-		 */
-		smp_send_stop();
-	} else {
-		/*
-		 * If we want to do crash dump after notifier calls and
-		 * kmsg_dump, we will need architecture dependent extra
-		 * works in addition to stopping other CPUs.
-		 */
-		crash_smp_send_stop();
-	}
+	panic_other_cpus_shutdown();
 
 	/*
 	 * Run any panic handlers, including those that might need to
@@ -370,7 +377,7 @@ void panic(const char *fmt, ...)
 	 *
 	 * Bypass the panic_cpu check and call __crash_kexec directly.
 	 */
-	if (_crash_kexec_post_notifiers)
+	if (crash_kexec_post_notifiers)
 		__crash_kexec(NULL);
 
 	console_unblank();
-- 
2.39.1