linux-kernel - [PATCH] printk: Fix panic log flush to serial console during kdump in PREEMPT

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Message-Id: <20250807112247.170127-1-cuiguoqi@kylinos.cn>
Date: Thu,  7 Aug 2025 19:22:47 +0800
From: cuiguoqi <cuiguoqi@...inos.cn>
To: catalin.marinas@....com,
	will@...nel.org,
	bigeasy@...utronix.de,
	clrkwllms@...nel.org,
	rostedt@...dmis.org
Cc: farbere@...zon.com,
	guoqi0226@....com,
	cuiguoqi@...inos.cn,
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	linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org,
	linux-rt-devel@...ts.linux.dev
Subject: [PATCH] printk: Fix panic log flush to serial console during kdump in PREEMPT_RT kernels

When a system running a real-time (PREEMPT_RT) kernel panics and triggers kdump,
the critical log messages (e.g., panic reason, stack traces) may fail to appear
on the serial console.

When kdump cannot be used properly, serial console logs are crucial,
whether for diagnosing kdump issues or troubleshooting the underlying problem.

This issue arises due to synchronization or deferred flushing of the printk buffer
in real-time contexts, where preemptible console locks or delayed workqueues prevent
timely log output before kexec transitions to the crash kernel.

The test results are as follows:
[  T197] Kernel panic - not syncing: sysrq triggered crash
[  T197] Call trace:
[  T197]  dump_backtrace+0x9c/0x120
[  T197]  show_stack+0x1c/0x30
[  T197]  dump_stack_lvl+0x34/0x88
[  T197]  dump_stack+0x14/0x20
[  T197]  panic+0x3c4/0x3f8
[  T197]  sysrq_handle_crash+0x20/0x28
[  T197]  __handle_sysrq+0xd4/0x1e0
[  T197]  write_sysrq_trigger+0x88/0x108
[  T197]  proc_reg_write+0x9c/0xf8
[  T197]  vfs_write+0xf4/0x450
[  T197]  ksys_write+0x70/0x100
[  T197]  __arm64_sys_write+0x20/0x30
[  T197]  invoke_syscall+0x48/0x110
[  T197]  el0_svc_common.constprop.0+0x44/0xe8
[  T197]  do_el0_svc+0x20/0x30
[  T197]  el0_svc+0x24/0x88
[  T197]  el0t_64_sync_handler+0xb8/0xc0
[  T197]  el0t_64_sync+0x14c/0x150
[  T197] SMP: stopping secondary CPUs
[  T197] Starting crashdump kernel...
[  T197] Bye!

Signed-off-by: cuiguoqi <cuiguoqi@...inos.cn>
---
 arch/arm64/kernel/machine_kexec.c | 4 ++++
 kernel/panic.c                    | 4 ++--
 2 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/kernel/machine_kexec.c b/arch/arm64/kernel/machine_kexec.c
index 6f121a0..66c7d90 100644
--- a/arch/arm64/kernel/machine_kexec.c
+++ b/arch/arm64/kernel/machine_kexec.c
@@ -24,6 +24,7 @@
 #include <asm/page.h>
 #include <asm/sections.h>
 #include <asm/trans_pgd.h>
+#include <linux/console.h>
 
 /**
  * kexec_image_info - For debugging output.
@@ -176,6 +177,9 @@ void machine_kexec(struct kimage *kimage)
 
 	pr_info("Bye!\n");
 
+	if (IS_ENABLED(CONFIG_PREEMPT_RT) && in_kexec_crash)
+		console_flush_on_panic(CONSOLE_FLUSH_PENDING);
+
 	local_daif_mask();
 
 	/*
diff --git a/kernel/panic.c b/kernel/panic.c
index 72fcbb5..e0ad0df 100644
--- a/kernel/panic.c
+++ b/kernel/panic.c
@@ -437,6 +437,8 @@ void vpanic(const char *fmt, va_list args)
 	 */
 	kgdb_panic(buf);
 
+	printk_legacy_allow_panic_sync();
+
 	/*
 	 * If we have crashed and we have a crash kernel loaded let it handle
 	 * everything else.
@@ -450,8 +452,6 @@ void vpanic(const char *fmt, va_list args)
 
 	panic_other_cpus_shutdown(_crash_kexec_post_notifiers);
 
-	printk_legacy_allow_panic_sync();
-
 	/*
 	 * Run any panic handlers, including those that might need to
 	 * add information to the kmsg dump output.
-- 
2.7.4