[PATCH v6 1/2] printk: Add function to replay kernel log on consoles

[Sreenath Vijayan]

Add a generic function console_replay_all() for replaying
the kernel log on consoles, in any context. It would allow
viewing the logs on an unresponsive terminal via sysrq.

Reuse the existing code from console_flush_on_panic() for
resetting the sequence numbers, by introducing a new helper
function __console_rewind_all(). It is safe to be called
under console_lock().

Try to acquire lock on the console subsystem without waiting.
If successful, reset the sequence number to oldest available
record on all consoles and call console_unlock() which will
automatically flush the messages to the consoles.

Suggested-by: John Ogness <john.ogness@xxxxxxxxxxxxx>
Suggested-by: Petr Mladek <pmladek@xxxxxxxx>
Signed-off-by: Shimoyashiki Taichi <taichi.shimoyashiki@xxxxxxxx>
Reviewed-by: John Ogness <john.ogness@xxxxxxxxxxxxx>
Signed-off-by: Sreenath Vijayan <sreenath.vijayan@xxxxxxxx>
---
 include/linux/printk.h |  4 +++
 kernel/printk/printk.c | 77 +++++++++++++++++++++++++++++-------------
 2 files changed, 57 insertions(+), 24 deletions(-)

diff --git a/include/linux/printk.h b/include/linux/printk.h
index 8ef499ab3c1e..8df2ac6c0e1f 100644
--- a/include/linux/printk.h
+++ b/include/linux/printk.h
@@ -192,6 +192,7 @@ void show_regs_print_info(const char *log_lvl);
 extern asmlinkage void dump_stack_lvl(const char *log_lvl) __cold;
 extern asmlinkage void dump_stack(void) __cold;
 void printk_trigger_flush(void);
+void console_replay_all(void);
 #else
 static inline __printf(1, 0)
 int vprintk(const char *s, va_list args)
@@ -271,6 +272,9 @@ static inline void dump_stack(void)
 static inline void printk_trigger_flush(void)
 {
 }
+static inline void console_replay_all(void)
+{
+}
 #endif
 
 #ifdef CONFIG_SMP
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index f2444b581e16..9e38cf0c1002 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -3134,6 +3134,40 @@ void console_unblank(void)
 		pr_flush(1000, true);
 }
 
+/*
+ * Rewind all consoles to the oldest available record.
+ *
+ * IMPORTANT: The function is safe only when called under
+ *            console_lock(). It is not enforced because
+ *            it is used as a best effort in panic().
+ */
+static void __console_rewind_all(void)
+{
+	struct console *c;
+	short flags;
+	int cookie;
+	u64 seq;
+
+	seq = prb_first_valid_seq(prb);
+
+	cookie = console_srcu_read_lock();
+	for_each_console_srcu(c) {
+		flags = console_srcu_read_flags(c);
+
+		if (flags & CON_NBCON) {
+			nbcon_seq_force(c, seq);
+		} else {
+			/*
+			 * This assignment is safe only when called under
+			 * console_lock(). On panic, legacy consoles are
+			 * only best effort.
+			 */
+			c->seq = seq;
+		}
+	}
+	console_srcu_read_unlock(cookie);
+}
+
 /**
  * console_flush_on_panic - flush console content on panic
  * @mode: flush all messages in buffer or just the pending ones
@@ -3162,30 +3196,8 @@ void console_flush_on_panic(enum con_flush_mode mode)
 	 */
 	console_may_schedule = 0;
 
-	if (mode == CONSOLE_REPLAY_ALL) {
-		struct console *c;
-		short flags;
-		int cookie;
-		u64 seq;
-
-		seq = prb_first_valid_seq(prb);
-
-		cookie = console_srcu_read_lock();
-		for_each_console_srcu(c) {
-			flags = console_srcu_read_flags(c);
-
-			if (flags & CON_NBCON) {
-				nbcon_seq_force(c, seq);
-			} else {
-				/*
-				 * This is an unsynchronized assignment. On
-				 * panic legacy consoles are only best effort.
-				 */
-				c->seq = seq;
-			}
-		}
-		console_srcu_read_unlock(cookie);
-	}
+	if (mode == CONSOLE_REPLAY_ALL)
+		__console_rewind_all();
 
 	console_flush_all(false, &next_seq, &handover);
 }
@@ -4259,6 +4271,23 @@ void kmsg_dump_rewind(struct kmsg_dump_iter *iter)
 }
 EXPORT_SYMBOL_GPL(kmsg_dump_rewind);
 
+/**
+ * console_replay_all - replay kernel log on consoles
+ *
+ * Try to obtain lock on console subsystem and replay all
+ * available records in printk buffer on the consoles.
+ * Does nothing if lock is not obtained.
+ *
+ * Context: Any context.
+ */
+void console_replay_all(void)
+{
+	if (console_trylock()) {
+		__console_rewind_all();
+		/* Consoles are flushed as part of console_unlock(). */
+		console_unlock();
+	}
+}
 #endif
 
 #ifdef CONFIG_SMP
-- 
2.25.1