Subject: [PATCH 20/48] printk: nbcon: Provide function to flush using write_atomic()
From: Thomas Gleixner <[email-masked]>
Date: Tue, 12 Sep 2023 12:00:08 +0000

Provide nbcon_atomic_flush_pending() to perform flushing of all
registered nbcon consoles using their write_atomic() callback.

Unlike console_flush_all(), nbcon_atomic_flush_pending() will
only flush up through the newest record at the time of the
call. This prevents a CPU from printing unbounded when other
CPUs are adding records.

Also unlike console_flush_all(), nbcon_atomic_flush_pending()
will fully flush one console before flushing the next. This
helps to guarantee that a block of pending records (such as
a stack trace in an emergency situation) can be printed
atomically at once before releasing console ownership.

nbcon_atomic_flush_pending() is safe in any context because it
uses write_atomic() and acquires with unsafe_takeover disabled.

Use it in console_flush_on_panic() before flushing legacy
consoles. The legacy write() callbacks are not fully safe when
oops_in_progress is set.

Co-developed-by: John Ogness <[email-masked]>
Signed-off-by: John Ogness <[email-masked]>
Signed-off-by: Thomas Gleixner (Intel) <[email-masked]>
Signed-off-by: Sebastian Andrzej Siewior <[email-masked]>
---
 kernel/printk/internal.h |    2 
 kernel/printk/nbcon.c    |  104 ++++++++++++++++++++++++++++++++++++++++++++++-
 kernel/printk/printk.c   |    2 
 3 files changed, 106 insertions(+), 2 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
@@ -84,6 +84,7 @@ void nbcon_seq_force(struct console *con
 bool nbcon_alloc(struct console *con);
 void nbcon_init(struct console *con);
 void nbcon_free(struct console *con);
+void nbcon_atomic_flush_pending(void);
 
 /*
  * Check if the given console is currently capable and allowed to print
@@ -138,6 +139,7 @@ static inline void nbcon_seq_force(struc
 static inline bool nbcon_alloc(struct console *con) { return false; }
 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 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
@@ -548,7 +548,6 @@ static struct printk_buffers panic_nbcon
  * in an unsafe state. Otherwise, on success the caller may assume
  * the console is not in an unsafe state.
  */
-__maybe_unused
 static bool nbcon_context_try_acquire(struct nbcon_context *ctxt)
 {
 	unsigned int cpu = smp_processor_id();
@@ -850,7 +849,6 @@ EXPORT_SYMBOL_GPL(nbcon_exit_unsafe);
  * When true is returned, @wctxt->ctxt.backlog indicates whether there are
  * still records pending in the ringbuffer,
  */
-__maybe_unused
 static bool nbcon_emit_next_record(struct nbcon_write_context *wctxt)
 {
 	struct nbcon_context *ctxt = &ACCESS_PRIVATE(wctxt, ctxt);
@@ -938,6 +936,108 @@ update_con:
 }
 
 /**
+ * __nbcon_atomic_flush_pending_con - Flush specified nbcon console using its
+ *					write_atomic() callback
+ * @con:			The nbcon console to flush
+ * @stop_seq:			Flush up until this record
+ *
+ * Return:	True if taken over while printing. Otherwise false.
+ *
+ * If flushing up to @stop_seq was not successful, it only makes sense for the
+ * caller to try again when true was returned. When false is returned, either
+ * there are no more records available to read or this context is not allowed
+ * to acquire the console.
+ */
+static bool __nbcon_atomic_flush_pending_con(struct console *con, u64 stop_seq)
+{
+	struct nbcon_write_context wctxt = { };
+	struct nbcon_context *ctxt = &ACCESS_PRIVATE(&wctxt, ctxt);
+
+	ctxt->console			= con;
+	ctxt->spinwait_max_us		= 2000;
+	ctxt->prio			= NBCON_PRIO_NORMAL;
+
+	if (!nbcon_context_try_acquire(ctxt))
+		return false;
+
+	while (nbcon_seq_read(con) < stop_seq) {
+		/*
+		 * 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;
+
+		if (!ctxt->backlog)
+			break;
+	}
+
+	nbcon_context_release(ctxt);
+
+	return false;
+}
+
+/**
+ * __nbcon_atomic_flush_pending - Flush all nbcon consoles using their
+ *					write_atomic() callback
+ * @stop_seq:			Flush up until this record
+ */
+static void __nbcon_atomic_flush_pending(u64 stop_seq)
+{
+	struct console *con;
+	bool should_retry;
+	int cookie;
+
+	do {
+		should_retry = false;
+
+		cookie = console_srcu_read_lock();
+		for_each_console_srcu(con) {
+			short flags = console_srcu_read_flags(con);
+			unsigned long irq_flags;
+
+			if (!(flags & CON_NBCON))
+				continue;
+
+			if (!console_is_usable(con, flags))
+				continue;
+
+			if (nbcon_seq_read(con) >= stop_seq)
+				continue;
+
+			/*
+			 * Atomic flushing does not use console driver
+			 * synchronization (i.e. it does not hold the port
+			 * lock for uart consoles). Therefore IRQs must be
+			 * disabled to avoid being interrupted and then
+			 * calling into a driver that will deadlock trying
+			 * to acquire console ownership.
+			 */
+			local_irq_save(irq_flags);
+
+			should_retry |= __nbcon_atomic_flush_pending_con(con, stop_seq);
+
+			local_irq_restore(irq_flags);
+		}
+		console_srcu_read_unlock(cookie);
+	} while (should_retry);
+}
+
+/**
+ * nbcon_atomic_flush_pending - Flush all nbcon consoles using their
+ *				write_atomic() callback
+ *
+ * Flush the backlog up through the currently newest record. Any new
+ * records added while flushing will not be flushed. This is to avoid
+ * one CPU printing unbounded because other CPUs continue to add records.
+ */
+void nbcon_atomic_flush_pending(void)
+{
+	__nbcon_atomic_flush_pending(prb_next_reserve_seq(prb));
+}
+
+/**
  * nbcon_alloc - Allocate buffers needed by the nbcon console
  * @con:	Console to allocate buffers for
  *
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
@@ -3177,6 +3177,8 @@ void console_flush_on_panic(enum con_flu
 		console_srcu_read_unlock(cookie);
 	}
 
+	nbcon_atomic_flush_pending();
+
 	console_flush_all(false, &next_seq, &handover);
 }