Merge tag 'printk-for-6.15' of git://git.kernel.org/pub/scm/linux/kernel/git/printk/linux

Pull printk updates from Petr Mladek:

 - New option "printk.debug_non_panic_cpus" allows to store printk
   messages from non-panic CPUs during panic. It might be useful when
   panic() fails. It is disabled by default because it increases the
   chance to see the messages printed before panic() and on the
   panic-CPU.

 - New build option "CONFIG_NULL_TTY_DEFAULT_CONSOLE" allows to build
   kernel without the virtual terminal support which prefers ttynull
   over serial console.

 - Do not unblank suspended consoles.

 - Some code clean up.

* tag 'printk-for-6.15' of git://git.kernel.org/pub/scm/linux/kernel/git/printk/linux:
  printk/panic: Add option to allow non-panic CPUs to write to the ring buffer.
  printk: Add an option to allow ttynull to be a default console device
  printk: Check CON_SUSPEND when unblanking a console
  printk: Rename console_start to console_resume
  printk: Rename console_stop to console_suspend
  printk: Rename resume_console to console_resume_all
  printk: Rename suspend_console to console_suspend_all

3 files changed, 53 insertions, 16 deletions

diff --git a/kernel/printk/internal.h b/kernel/printk/internal.h
index a91bdf802967..48a24e7b309d 100644
--- a/kernel/printk/internal.h
+++ b/kernel/printk/internal.h
@@ -64,6 +64,7 @@ struct dev_printk_info;
 
 extern struct printk_ringbuffer *prb;
 extern bool printk_kthreads_running;
+extern bool debug_non_panic_cpus;
 
 __printf(4, 0)
 int vprintk_store(int facility, int level,
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 057db78876cd..1eea80d0648e 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2375,6 +2375,22 @@ void printk_legacy_allow_panic_sync(void)
 	}
 }
 
+bool __read_mostly debug_non_panic_cpus;
+
+#ifdef CONFIG_PRINTK_CALLER
+static int __init debug_non_panic_cpus_setup(char *str)
+{
+	debug_non_panic_cpus = true;
+	pr_info("allow messages from non-panic CPUs in panic()\n");
+
+	return 0;
+}
+early_param("debug_non_panic_cpus", debug_non_panic_cpus_setup);
+module_param(debug_non_panic_cpus, bool, 0644);
+MODULE_PARM_DESC(debug_non_panic_cpus,
+		 "allow messages from non-panic CPUs in panic()");
+#endif
+
 asmlinkage int vprintk_emit(int facility, int level,
 			    const struct dev_printk_info *dev_info,
 			    const char *fmt, va_list args)
@@ -2391,7 +2407,9 @@ asmlinkage int vprintk_emit(int facility, int level,
 	 * non-panic CPUs are generating any messages, they will be
 	 * silently dropped.
 	 */
-	if (other_cpu_in_panic() && !panic_triggering_all_cpu_backtrace)
+	if (other_cpu_in_panic() &&
+	    !debug_non_panic_cpus &&
+	    !panic_triggering_all_cpu_backtrace)
 		return 0;
 
 	printk_get_console_flush_type(&ft);
@@ -2731,11 +2749,11 @@ module_param_named(console_no_auto_verbose, printk_console_no_auto_verbose, bool
 MODULE_PARM_DESC(console_no_auto_verbose, "Disable console loglevel raise to highest on oops/panic/etc");
 
 /**
- * suspend_console - suspend the console subsystem
+ * console_suspend_all - suspend the console subsystem
  *
  * This disables printk() while we go into suspend states
  */
-void suspend_console(void)
+void console_suspend_all(void)
 {
 	struct console *con;
 
@@ -2758,7 +2776,7 @@ void suspend_console(void)
 	synchronize_srcu(&console_srcu);
 }
 
-void resume_console(void)
+void console_resume_all(void)
 {
 	struct console_flush_type ft;
 	struct console *con;
@@ -3340,7 +3358,12 @@ void console_unblank(void)
 	 */
 	cookie = console_srcu_read_lock();
 	for_each_console_srcu(c) {
-		if ((console_srcu_read_flags(c) & CON_ENABLED) && c->unblank) {
+		short flags = console_srcu_read_flags(c);
+
+		if (flags & CON_SUSPENDED)
+			continue;
+
+		if ((flags & CON_ENABLED) && c->unblank) {
 			found_unblank = true;
 			break;
 		}
@@ -3377,7 +3400,12 @@ void console_unblank(void)
 
 	cookie = console_srcu_read_lock();
 	for_each_console_srcu(c) {
-		if ((console_srcu_read_flags(c) & CON_ENABLED) && c->unblank)
+		short flags = console_srcu_read_flags(c);
+
+		if (flags & CON_SUSPENDED)
+			continue;
+
+		if ((flags & CON_ENABLED) && c->unblank)
 			c->unblank();
 	}
 	console_srcu_read_unlock(cookie);
@@ -3495,10 +3523,10 @@ struct tty_driver *console_device(int *index)
 
 /*
  * Prevent further output on the passed console device so that (for example)
- * serial drivers can disable console output before suspending a port, and can
+ * serial drivers can suspend console output before suspending a port, and can
  * re-enable output afterwards.
  */
-void console_stop(struct console *console)
+void console_suspend(struct console *console)
 {
 	__pr_flush(console, 1000, true);
 	console_list_lock();
@@ -3513,9 +3541,9 @@ void console_stop(struct console *console)
 	 */
 	synchronize_srcu(&console_srcu);
 }
-EXPORT_SYMBOL(console_stop);
+EXPORT_SYMBOL(console_suspend);
 
-void console_start(struct console *console)
+void console_resume(struct console *console)
 {
 	struct console_flush_type ft;
 	bool is_nbcon;
@@ -3540,7 +3568,7 @@ void console_start(struct console *console)
 
 	__pr_flush(console, 1000, true);
 }
-EXPORT_SYMBOL(console_start);
+EXPORT_SYMBOL(console_resume);
 
 #ifdef CONFIG_PRINTK
 static int unregister_console_locked(struct console *console);
@@ -4275,6 +4303,11 @@ void __init console_init(void)
 	initcall_t call;
 	initcall_entry_t *ce;
 
+#ifdef CONFIG_NULL_TTY_DEFAULT_CONSOLE
+	if (!console_set_on_cmdline)
+		add_preferred_console("ttynull", 0, NULL);
+#endif
+
 	/* Setup the default TTY line discipline. */
 	n_tty_init();
 
diff --git a/kernel/printk/printk_ringbuffer.c b/kernel/printk/printk_ringbuffer.c
index 88e8f3a61922..d9fb053cff67 100644
--- a/kernel/printk/printk_ringbuffer.c
+++ b/kernel/printk/printk_ringbuffer.c
@@ -2133,9 +2133,9 @@ static bool _prb_read_valid(struct printk_ringbuffer *rb, u64 *seq,
 			 * there may be other finalized records beyond that
 			 * need to be printed for a panic situation. If this
 			 * is the panic CPU, skip this
-			 * non-existent/non-finalized record unless it is
-			 * at or beyond the head, in which case it is not
-			 * possible to continue.
+			 * non-existent/non-finalized record unless non-panic
+			 * CPUs are still running and their debugging is
+			 * explicitly enabled.
 			 *
 			 * Note that new messages printed on panic CPU are
 			 * finalized when we are here. The only exception
@@ -2143,10 +2143,13 @@ static bool _prb_read_valid(struct printk_ringbuffer *rb, u64 *seq,
 			 * But it would have the sequence number returned
 			 * by "prb_next_reserve_seq() - 1".
 			 */
-			if (this_cpu_in_panic() && ((*seq + 1) < prb_next_reserve_seq(rb)))
+			if (this_cpu_in_panic() &&
+			    (!debug_non_panic_cpus || legacy_allow_panic_sync) &&
+			    ((*seq + 1) < prb_next_reserve_seq(rb))) {
 				(*seq)++;
-			else
+			} else {
 				return false;
+			}
 		}
 	}