From: John Ogness <john.ogness@linutronix.de>
Date: Mon, 25 Mar 2024 21:00:40 +0000
Subject: [PATCH 37/46] printk: nbcon: Show replay message on takeover

An emergency or panic context can takeover console ownership
while the current owner was printing a printk message. The
atomic printer will re-print the message that the previous
owner was printing. However, this can look confusing to the
user and may even seem as though a message was lost.

  [3430014.1
  [3430014.181123] usb 1-2: Product: USB Audio

Add a new field @nbcon_prev_seq to struct console to track
the sequence number to print that was assigned to the previous
console owner. If this matches the sequence number to print
that the current owner is assigned, then a takeover must have
occurred. In this case, print an additional message to inform
the user that the previous message is being printed again.

  [3430014.1
  ** replaying previous printk message **
  [3430014.181123] usb 1-2: Product: USB Audio

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 include/linux/console.h  |    3 +++
 kernel/printk/internal.h |    1 +
 kernel/printk/nbcon.c    |   24 ++++++++++++++++++++++++
 kernel/printk/printk.c   |   19 +++++++++++++++++++
 4 files changed, 47 insertions(+)

@ include/linux/console.h:328 @ struct nbcon_write_context {
  * @nbcon_state:	State for nbcon consoles
  * @nbcon_seq:		Sequence number of the next record for nbcon to print
  * @nbcon_driver_ctxt:	Context available for driver non-printing operations
+ * @nbcon_prev_seq:	Seq num the previous nbcon owner was assigned to print
  * @pbufs:		Pointer to nbcon private buffer
  * @kthread:		Printer kthread for this console
  * @rcuwait:		RCU-safe wait object for @kthread waking
@ include/linux/console.h:449 @ struct console {
 	atomic_t		__private nbcon_state;
 	atomic_long_t		__private nbcon_seq;
 	struct nbcon_context	__private nbcon_driver_ctxt;
+	atomic_long_t           __private nbcon_prev_seq;
+
 	struct printk_buffers	*pbufs;
 	struct task_struct	*kthread;
 	struct rcuwait		rcuwait;
--- a/kernel/printk/internal.h
+++ b/kernel/printk/internal.h
@ include/linux/console.h:234 @ bool printk_get_next_message(struct prin
 
 #ifdef CONFIG_PRINTK
 void console_prepend_dropped(struct printk_message *pmsg, unsigned long dropped);
+void console_prepend_replay(struct printk_message *pmsg);
 #endif
--- a/kernel/printk/nbcon.c
+++ b/kernel/printk/nbcon.c
@ include/linux/console.h:869 @ static bool nbcon_emit_next_record(struc
 	unsigned long con_dropped;
 	struct nbcon_state cur;
 	unsigned long dropped;
+	unsigned long ulseq;
 
 	/*
 	 * The printk buffers are filled within an unsafe section. This
@ include/linux/console.h:895 @ static bool nbcon_emit_next_record(struc
 	if (dropped && !is_extended)
 		console_prepend_dropped(&pmsg, dropped);
 
+	/*
+	 * If the previous owner was assigned the same record, this context
+	 * has taken over ownership and is replaying the record. Prepend a
+	 * message to let the user know the record is replayed.
+	 */
+	ulseq = atomic_long_read(&ACCESS_PRIVATE(con, nbcon_prev_seq));
+	if (__ulseq_to_u64seq(prb, ulseq) == pmsg.seq) {
+		console_prepend_replay(&pmsg);
+	} else {
+		/*
+		 * Ensure this context is still the owner before trying to
+		 * update @nbcon_prev_seq. Otherwise the value in @ulseq may
+		 * not be from the previous owner.
+		 */
+		nbcon_state_read(con, &cur);
+		if (!nbcon_context_can_proceed(ctxt, &cur))
+			return false;
+
+		atomic_long_try_cmpxchg(&ACCESS_PRIVATE(con, nbcon_prev_seq), &ulseq,
+					__u64seq_to_ulseq(pmsg.seq));
+	}
+
 	if (!nbcon_context_exit_unsafe(ctxt))
 		return false;
 
@ include/linux/console.h:1637 @ void nbcon_init(struct console *con, u64
 	rcuwait_init(&con->rcuwait);
 	init_irq_work(&con->irq_work, nbcon_irq_work);
 	nbcon_seq_force(con, init_seq);
+	atomic_long_set(&ACCESS_PRIVATE(con, nbcon_prev_seq), -1UL);
 	nbcon_state_set(con, &state);
 	nbcon_kthread_create(con);
 }
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@ include/linux/console.h:2871 @ void console_prepend_dropped(struct prin
 }
 
 /*
+ * Prepend the message in @pmsg->pbufs->outbuf with a "replay message".
+ * @pmsg->outbuf_len is updated appropriately.
+ *
+ * @pmsg is the printk message to prepend.
+ */
+void console_prepend_replay(struct printk_message *pmsg)
+{
+	struct printk_buffers *pbufs = pmsg->pbufs;
+	const size_t scratchbuf_sz = sizeof(pbufs->scratchbuf);
+	char *scratchbuf = &pbufs->scratchbuf[0];
+	size_t len;
+
+	len = scnprintf(scratchbuf, scratchbuf_sz,
+			"** replaying previous printk message **\n");
+
+	__console_prepend_scratch(pmsg, len);
+}
+
+/*
  * Read and format the specified record (or a later record if the specified
  * record is not available).
  *