[PATCH v4 1/2] printk: Add function to dump printk buffer directly to consoles

[Sreenath Vijayan <sreenath.vijayan@xxxxxxxx>]

It is useful to be able to dump the printk buffer directly to
consoles in some situations so as to not flood the buffer.
To do this, we reuse the CONSOLE_REPLAY_ALL mode code in
console_flush_on_panic() by moving the code to a helper function
console_rewind_all(). This is done because console_flush_on_panic()
sets console_may_schedule to 0 but this should not be done in our
case. Then console_rewind_all() is called from the new function
dump_printk_buffer() with console lock held to set the console
sequence number to oldest record in the buffer for all consoles.
Releasing the console lock will flush the contents of printk buffer
to the consoles.

Suggested-by: John Ogness <john.ogness@xxxxxxxxxxxxx>
Signed-off-by: Sreenath Vijayan <sreenath.vijayan@xxxxxxxx>
Signed-off-by: Shimoyashiki Taichi <taichi.shimoyashiki@xxxxxxxx>
---
 include/linux/printk.h |  4 +++
 kernel/printk/printk.c | 61 +++++++++++++++++++++++++-----------------
 2 files changed, 41 insertions(+), 24 deletions(-)

diff --git a/include/linux/printk.h b/include/linux/printk.h
index 8ef499ab3c1e..861ff5a545ff 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 dump_printk_buffer(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 void dump_printk_buffer(void)
+{
+}
 #endif
 
 #ifdef CONFIG_SMP
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index f2444b581e16..b05ca9f98e53 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -3134,6 +3134,32 @@ void console_unblank(void)
 		pr_flush(1000, true);
 }
 
+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 is an unsynchronized assignment. 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 +3188,8 @@ 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 +4263,15 @@ void kmsg_dump_rewind(struct kmsg_dump_iter *iter)
 }
 EXPORT_SYMBOL_GPL(kmsg_dump_rewind);
 
+/**
+ * Dump the printk ring buffer directly to consoles
+ */
+void dump_printk_buffer(void)
+{
+	console_lock();
+	console_rewind_all();
+	console_unlock();
+}
 #endif
 
 #ifdef CONFIG_SMP
-- 
2.43.0