Subject: [PATCH 22/48] printk: nbcon: Use nbcon consoles in console_flush_all()
From: John Ogness <[email-masked]>
Date: Tue, 19 Sep 2023 14:33:27 +0000

Allow nbcon consoles to print messages in the legacy printk()
caller context (printing via unlock) by integrating them into
console_flush_all(). The write_atomic() callback is used for
printing.

Provide nbcon_legacy_emit_next_record(), which acts as the
nbcon variant of console_emit_next_record(). Call this variant
within console_flush_all() for nbcon consoles. Since nbcon
consoles use their own @nbcon_seq variable to track the next
record to print, this also must be appropriately handled.

Signed-off-by: John Ogness <[email-masked]>
Signed-off-by: Sebastian Andrzej Siewior <[email-masked]>
---
 kernel/printk/internal.h |    6 +++
 kernel/printk/nbcon.c    |   77 +++++++++++++++++++++++++++++++++++++++++++++++
 kernel/printk/printk.c   |   17 +++++++---
 3 files changed, 95 insertions(+), 5 deletions(-)

Index: linux-6.8.2-rt10/kernel/printk/internal.h
===================================================================
--- linux-6.8.2-rt10.orig/kernel/printk/internal.h
+++ linux-6.8.2-rt10/kernel/printk/internal.h
@@ -78,6 +78,8 @@ void defer_console_output(void);
 
 u16 printk_parse_prefix(const char *text, int *level,
 			enum printk_info_flags *flags);
+void console_lock_spinning_enable(void);
+int console_lock_spinning_disable_and_check(int cookie);
 
 u64 nbcon_seq_read(struct console *con);
 void nbcon_seq_force(struct console *con, u64 seq);
@@ -85,6 +87,8 @@ bool nbcon_alloc(struct console *con);
 void nbcon_init(struct console *con);
 void nbcon_free(struct console *con);
 void nbcon_atomic_flush_pending(void);
+bool nbcon_legacy_emit_next_record(struct console *con, bool *handover,
+				   int cookie);
 
 /*
  * Check if the given console is currently capable and allowed to print
@@ -140,6 +144,8 @@ static inline bool nbcon_alloc(struct co
 static inline void nbcon_init(struct console *con) { }
 static inline void nbcon_free(struct console *con) { }
 static inline void nbcon_atomic_flush_pending(void) { }
+static inline bool nbcon_legacy_emit_next_record(struct console *con, bool *handover,
+						 int cookie) { return false; }
 
 static inline bool console_is_usable(struct console *con, short flags) { return false; }
 
Index: linux-6.8.2-rt10/kernel/printk/nbcon.c
===================================================================
--- linux-6.8.2-rt10.orig/kernel/printk/nbcon.c
+++ linux-6.8.2-rt10/kernel/printk/nbcon.c
@@ -541,6 +541,7 @@ static struct printk_buffers panic_nbcon
  * nbcon_context_try_acquire - Try to acquire nbcon console
  * @ctxt:	The context of the caller
  *
+ * Context:	Any context which could not be migrated to another CPU.
  * Return:	True if the console was acquired. False otherwise.
  *
  * If the caller allowed an unsafe hostile takeover, on success the
@@ -936,6 +937,82 @@ update_con:
 }
 
 /**
+ * nbcon_atomic_emit_one - Print one record for an nbcon console using the
+ *				write_atomic() callback
+ * @wctxt:	An initialized write context struct to use for this context
+ *
+ * Return:	False if it is known there are no more records to print,
+ *		otherwise true.
+ *
+ * This is an internal helper to handle the locking of the console before
+ * calling nbcon_emit_next_record().
+ */
+static bool nbcon_atomic_emit_one(struct nbcon_write_context *wctxt)
+{
+	struct nbcon_context *ctxt = &ACCESS_PRIVATE(wctxt, ctxt);
+
+	if (!nbcon_context_try_acquire(ctxt))
+		return true;
+
+	/*
+	 * nbcon_emit_next_record() returns false when the console was
+	 * handed over or taken over. In both cases the context is no
+	 * longer valid.
+	 */
+	if (!nbcon_emit_next_record(wctxt))
+		return true;
+
+	nbcon_context_release(ctxt);
+
+	return ctxt->backlog;
+}
+
+/**
+ * nbcon_legacy_emit_next_record - Print one record for an nbcon console
+ *					in legacy contexts
+ * @con:	The console to print on
+ * @handover:	Will be set to true if a printk waiter has taken over the
+ *		console_lock, in which case the caller is no longer holding
+ *		both the console_lock and the SRCU read lock. Otherwise it
+ *		is set to false.
+ * @cookie:	The cookie from the SRCU read lock.
+ *
+ * Context:	Any context except NMI.
+ * Return:	False if the given console has no next record to print,
+ *		otherwise true.
+ *
+ * This function is meant to be called by console_flush_all() to print records
+ * on nbcon consoles from legacy context (printing via console unlocking).
+ * Essentially it is the nbcon version of console_emit_next_record().
+ */
+bool nbcon_legacy_emit_next_record(struct console *con, bool *handover,
+				   int cookie)
+{
+	struct nbcon_write_context wctxt = { };
+	struct nbcon_context *ctxt = &ACCESS_PRIVATE(&wctxt, ctxt);
+	unsigned long flags;
+	bool progress;
+
+	*handover = false;
+
+	/* Use the same procedure as console_emit_next_record(). */
+	printk_safe_enter_irqsave(flags);
+	console_lock_spinning_enable();
+	stop_critical_timings();
+
+	ctxt->console	= con;
+	ctxt->prio	= NBCON_PRIO_NORMAL;
+
+	progress = nbcon_atomic_emit_one(&wctxt);
+
+	start_critical_timings();
+	*handover = console_lock_spinning_disable_and_check(cookie);
+	printk_safe_exit_irqrestore(flags);
+
+	return progress;
+}
+
+/**
  * __nbcon_atomic_flush_pending_con - Flush specified nbcon console using its
  *					write_atomic() callback
  * @con:			The nbcon console to flush
Index: linux-6.8.2-rt10/kernel/printk/printk.c
===================================================================
--- linux-6.8.2-rt10.orig/kernel/printk/printk.c
+++ linux-6.8.2-rt10/kernel/printk/printk.c
@@ -1871,7 +1871,7 @@ static bool console_waiter;
  * there may be a waiter spinning (like a spinlock). Also it must be
  * ready to hand over the lock at the end of the section.
  */
-static void console_lock_spinning_enable(void)
+void console_lock_spinning_enable(void)
 {
 	/*
 	 * Do not use spinning in panic(). The panic CPU wants to keep the lock.
@@ -1910,7 +1910,7 @@ lockdep:
  *
  * Return: 1 if the lock rights were passed, 0 otherwise.
  */
-static int console_lock_spinning_disable_and_check(int cookie)
+int console_lock_spinning_disable_and_check(int cookie)
 {
 	int waiter;
 
@@ -2956,13 +2956,20 @@ static bool console_flush_all(bool do_co
 		cookie = console_srcu_read_lock();
 		for_each_console_srcu(con) {
 			short flags = console_srcu_read_flags(con);
+			u64 printk_seq;
 			bool progress;
 
 			if (!console_is_usable(con, flags))
 				continue;
 			any_usable = true;
 
-			progress = console_emit_next_record(con, handover, cookie);
+			if (flags & CON_NBCON) {
+				progress = nbcon_legacy_emit_next_record(con, handover, cookie);
+				printk_seq = nbcon_seq_read(con);
+			} else {
+				progress = console_emit_next_record(con, handover, cookie);
+				printk_seq = con->seq;
+			}
 
 			/*
 			 * If a handover has occurred, the SRCU read lock
@@ -2972,8 +2979,8 @@ static bool console_flush_all(bool do_co
 				return false;
 
 			/* Track the next of the highest seq flushed. */
-			if (con->seq > *next_seq)
-				*next_seq = con->seq;
+			if (printk_seq > *next_seq)
+				*next_seq = printk_seq;
 
 			if (!progress)
 				continue;