[PATCH printk v2 14/26] printk: nbcon: Provide function to flush using write_atomic()

[John Ogness <john.ogness@linutronix.de>]

From: Thomas Gleixner <tglx@linutronix.de>

Provide nbcon_atomic_flush_all() to perform flushing of all
registered nbcon consoles using their write_atomic() callback.
Like with legacy consoles, the nbcon consoles are flushed one
record per console. This allows all nbcon consoles to print
lines pseudo-simultaneously, rather than one console waiting
for the full ringbuffer to dump to another console before
printing anything.

Unlike console_flush_all(), nbcon_atomic_flush_all() 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.

Perform nbcon console atomic flushing in
console_flush_on_panic(). This function is not only used in
panic() but also other locations where there may be stored
messages that need to be flushed.

Co-developed-by: John Ogness <john.ogness@linutronix.de>
Signed-off-by: John Ogness <john.ogness@linutronix.de>
Signed-off-by: Thomas Gleixner (Intel) <tglx@linutronix.de>
---
 kernel/printk/internal.h |   2 +
 kernel/printk/nbcon.c    | 100 ++++++++++++++++++++++++++++++++++++++-
 kernel/printk/printk.c   |   2 +
 3 files changed, 102 insertions(+), 2 deletions(-)

diff --git a/kernel/printk/internal.h b/kernel/printk/internal.h
index 6780911fa8f2..d9a5205692fc 100644
--- a/kernel/printk/internal.h
+++ b/kernel/printk/internal.h
@@ -77,6 +77,7 @@ void nbcon_seq_force(struct console *con, u64 seq);
 bool nbcon_alloc(struct console *con);
 void nbcon_init(struct console *con);
 void nbcon_free(struct console *con);
+void nbcon_atomic_flush_all(void);
 
 /*
  * Check if the given console is currently capable and allowed to print
@@ -131,6 +132,7 @@ static inline void nbcon_seq_force(struct console *con, u64 seq) { }
 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_all(void) { }
 
 static inline bool console_is_usable(struct console *con, short flags) { return false; }
 
diff --git a/kernel/printk/nbcon.c b/kernel/printk/nbcon.c
index 02e8fdc1ea43..2eb2929c1027 100644
--- a/kernel/printk/nbcon.c
+++ b/kernel/printk/nbcon.c
@@ -539,7 +539,6 @@ static struct printk_buffers panic_nbcon_pbufs;
  * 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();
@@ -841,7 +840,6 @@ 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);
@@ -930,6 +928,104 @@ static bool nbcon_emit_next_record(struct nbcon_write_context *wctxt)
 	return nbcon_context_exit_unsafe(ctxt);
 }
 
+/**
+ * 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 the given console could not print a record or 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 false;
+
+	/*
+	 * 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 false;
+
+	nbcon_context_release(ctxt);
+
+	return ctxt->backlog;
+}
+
+/**
+ * __nbcon_atomic_flush_all - Flush all nbcon consoles using their
+ *					write_atomic() callback
+ * @stop_seq:			Flush up until this record
+ */
+static void __nbcon_atomic_flush_all(u64 stop_seq)
+{
+	struct nbcon_write_context wctxt = { };
+	struct nbcon_context *ctxt = &ACCESS_PRIVATE(&wctxt, ctxt);
+	struct console *con;
+	bool any_progress;
+	int cookie;
+
+	do {
+		any_progress = 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;
+
+			memset(ctxt, 0, sizeof(*ctxt));
+			ctxt->console			= con;
+			ctxt->spinwait_max_us		= 2000;
+			ctxt->prio			= NBCON_PRIO_NORMAL;
+
+			/*
+			 * 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
+			 * acquire console ownership.
+			 */
+			local_irq_save(irq_flags);
+
+			any_progress |= nbcon_atomic_emit_one(&wctxt);
+
+			local_irq_restore(irq_flags);
+		}
+		console_srcu_read_unlock(cookie);
+	} while (any_progress);
+}
+
+/**
+ * nbcon_atomic_flush_all - 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_all(void)
+{
+	__nbcon_atomic_flush_all(prb_next_reserve_seq(prb));
+}
+
 /**
  * nbcon_alloc - Allocate buffers needed by the nbcon console
  * @con:	Console to allocate buffers for
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 9d56ce5837f9..ea170ade4d42 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -3169,6 +3169,8 @@ void console_flush_on_panic(enum con_flush_mode mode)
 		console_srcu_read_unlock(cookie);
 	}
 
+	nbcon_atomic_flush_all();
+
 	console_flush_all(false, &next_seq, &handover);
 }
 
-- 
2.39.2