linux-kernel - [PATCH] kgdb: Flush console before entering kgdb on panic

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Date:   Tue, 22 Aug 2023 13:19:46 -0700
From:   Douglas Anderson <dianders@...omium.org>
To:     Daniel Thompson <daniel.thompson@...aro.org>
Cc:     Petr Mladek <pmladek@...e.com>,
        Douglas Anderson <dianders@...omium.org>,
        Jason Wessel <jason.wessel@...driver.com>,
        kgdb-bugreport@...ts.sourceforge.net, linux-kernel@...r.kernel.org
Subject: [PATCH] kgdb: Flush console before entering kgdb on panic

When entering kdb/kgdb on a kernel panic, it was be observed that the
console isn't flushed before the `kdb` prompt came up. Specifically,
when using the buddy lockup detector on arm64 and running:
  echo HARDLOCKUP > /sys/kernel/debug/provoke-crash/DIRECT

I could see:
  [   26.161099] lkdtm: Performing direct entry HARDLOCKUP
  [   32.499881] watchdog: Watchdog detected hard LOCKUP on cpu 6
  [   32.552865] Sending NMI from CPU 5 to CPUs 6:
  [   32.557359] NMI backtrace for cpu 6
  ... [backtrace for cpu 6] ...
  [   32.558353] NMI backtrace for cpu 5
  ... [backtrace for cpu 5] ...
  [   32.867471] Sending NMI from CPU 5 to CPUs 0-4,7:
  [   32.872321] NMI backtrace forP cpuANC: Hard LOCKUP

  Entering kdb (current=..., pid 0) on processor 5 due to Keyboard Entry
  [5]kdb>

As you can see, backtraces for the other CPUs start printing and get
interleaved with the kdb PANIC print.

Let's replicate the commands to flush the console in the kdb panic
entry point to avoid this.

Signed-off-by: Douglas Anderson <dianders@...omium.org>
---

 kernel/debug/debug_core.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/debug/debug_core.c b/kernel/debug/debug_core.c
index d5e9ccde3ab8..3a904d8697c8 100644
--- a/kernel/debug/debug_core.c
+++ b/kernel/debug/debug_core.c
@@ -1006,6 +1006,9 @@ void kgdb_panic(const char *msg)
 	if (panic_timeout)
 		return;
 
+	debug_locks_off();
+	console_flush_on_panic(CONSOLE_FLUSH_PENDING);
+
 	if (dbg_kdb_mode)
 		kdb_printf("PANIC: %s\n", msg);
 
-- 
2.42.0.rc1.204.g551eb34607-goog