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Message-Id: <20250930-nbcon-kgdboc-v5-3-8125893cfb4f@suse.com>
Date: Tue, 30 Sep 2025 14:21:09 -0300
From: Marcos Paulo de Souza <mpdesouza@...e.com>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
Petr Mladek <pmladek@...e.com>, Steven Rostedt <rostedt@...dmis.org>,
John Ogness <john.ogness@...utronix.de>,
Sergey Senozhatsky <senozhatsky@...omium.org>,
Jason Wessel <jason.wessel@...driver.com>,
Daniel Thompson <danielt@...nel.org>,
Douglas Anderson <dianders@...omium.org>
Cc: linux-kernel@...r.kernel.org, kgdb-bugreport@...ts.sourceforge.net,
Marcos Paulo de Souza <mpdesouza@...e.com>
Subject: [PATCH v5 3/5] printk: nbcon: Allow KDB to acquire the NBCON
context
KDB can interrupt any console to execute the "mirrored printing" at any
time, so add an exception to nbcon_context_try_acquire_direct to allow
to get the context if the current CPU is the same as kdb_printf_cpu.
This change will be necessary for the next patch, which fixes
kdb_msg_write to work with NBCON consoles by calling ->write_atomic on
such consoles. But to print it first needs to acquire the ownership of
the console, so nbcon_context_try_acquire_direct is fixed here.
Signed-off-by: Marcos Paulo de Souza <mpdesouza@...e.com>
---
include/linux/kdb.h | 15 +++++++++++++++
kernel/printk/nbcon.c | 6 +++++-
2 files changed, 20 insertions(+), 1 deletion(-)
diff --git a/include/linux/kdb.h b/include/linux/kdb.h
index ecbf819deeca118f27e98bf71bb37dd27a257ebb..36b46c85733fe1df28cde7760e5c26e96de40c05 100644
--- a/include/linux/kdb.h
+++ b/include/linux/kdb.h
@@ -207,11 +207,26 @@ static inline const char *kdb_walk_kallsyms(loff_t *pos)
/* Dynamic kdb shell command registration */
extern int kdb_register(kdbtab_t *cmd);
extern void kdb_unregister(kdbtab_t *cmd);
+
+/* Return true when KDB as locked for printing a message on this CPU. */
+static inline
+bool kdb_printf_on_this_cpu(void)
+{
+ /*
+ * We can use raw_smp_processor_id() here because the task could
+ * not get migrated when KDB has locked for printing on this CPU.
+ */
+ return unlikely(READ_ONCE(kdb_printf_cpu) == raw_smp_processor_id());
+}
+
#else /* ! CONFIG_KGDB_KDB */
static inline __printf(1, 2) int kdb_printf(const char *fmt, ...) { return 0; }
static inline void kdb_init(int level) {}
static inline int kdb_register(kdbtab_t *cmd) { return 0; }
static inline void kdb_unregister(kdbtab_t *cmd) {}
+
+static inline bool kdb_printf_on_this_cpu(void) { return false };
+
#endif /* CONFIG_KGDB_KDB */
enum {
KDB_NOT_INITIALIZED,
diff --git a/kernel/printk/nbcon.c b/kernel/printk/nbcon.c
index c23abed5933527cb7c6bcc42057fadbb44a43446..6e4641350fe8985438f53bcd32f3adf72d9d6835 100644
--- a/kernel/printk/nbcon.c
+++ b/kernel/printk/nbcon.c
@@ -10,6 +10,7 @@
#include <linux/export.h>
#include <linux/init.h>
#include <linux/irqflags.h>
+#include <linux/kdb.h>
#include <linux/kthread.h>
#include <linux/minmax.h>
#include <linux/panic.h>
@@ -249,13 +250,16 @@ static int nbcon_context_try_acquire_direct(struct nbcon_context *ctxt,
* since all non-panic CPUs are stopped during panic(), it
* is safer to have them avoid gaining console ownership.
*
- * If this acquire is a reacquire (and an unsafe takeover
+ * One exception is when kdb has locked for printing on this CPU.
+ *
+ * Second exception is a reacquire (and an unsafe takeover
* has not previously occurred) then it is allowed to attempt
* a direct acquire in panic. This gives console drivers an
* opportunity to perform any necessary cleanup if they were
* interrupted by the panic CPU while printing.
*/
if (panic_on_other_cpu() &&
+ !kdb_printf_on_this_cpu() &&
(!is_reacquire || cur->unsafe_takeover)) {
return -EPERM;
}
--
2.51.0
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