[PATCH printk v3 08/19] printk: nbcon: Add context to usable() and emit()

[John Ogness]

The nbcon consoles will have two callbacks to be used for
different contexts. In order to determine if an nbcon console
is usable, console_is_usable() must know if it is a context
that will need to use the optional write_atomic() callback.
Also, nbcon_emit_next_record() must know which callback it
needs to call.

Add an extra parameter @use_atomic to console_is_usable() and
nbcon_emit_next_record() to specify this.

Since so far only the write_atomic() callback exists,
@use_atomic is set to true for all call sites.

For legacy consoles, @use_atomic is not used.

Signed-off-by: John Ogness <john.ogness@xxxxxxxxxxxxx>
---
 include/linux/console.h  |  2 +-
 kernel/printk/internal.h |  8 +++++---
 kernel/printk/nbcon.c    | 25 +++++++++++++++++++------
 kernel/printk/printk.c   |  6 +++---
 4 files changed, 28 insertions(+), 13 deletions(-)

diff --git a/include/linux/console.h b/include/linux/console.h
index 38ef6e64da19..5e9285e886f5 100644
--- a/include/linux/console.h
+++ b/include/linux/console.h
@@ -349,7 +349,7 @@ struct console {
 	/**
 	 * @write_atomic:
 	 *
-	 * NBCON callback to write out text in any context.
+	 * NBCON callback to write out text in any context. (Optional)
 	 *
 	 * This callback is called with the console already acquired. However,
 	 * a higher priority context is allowed to take it over by default.
diff --git a/kernel/printk/internal.h b/kernel/printk/internal.h
index ccdb81dc18f0..8211623aee9b 100644
--- a/kernel/printk/internal.h
+++ b/kernel/printk/internal.h
@@ -98,7 +98,7 @@ bool nbcon_legacy_emit_next_record(struct console *con, bool *handover,
  * which can also play a role in deciding if @con can be used to print
  * records.
  */
-static inline bool console_is_usable(struct console *con, short flags)
+static inline bool console_is_usable(struct console *con, short flags, bool use_atomic)
 {
 	if (!(flags & CON_ENABLED))
 		return false;
@@ -107,7 +107,8 @@ static inline bool console_is_usable(struct console *con, short flags)
 		return false;
 
 	if (flags & CON_NBCON) {
-		if (!con->write_atomic)
+		/* The write_atomic() callback is optional. */
+		if (use_atomic && !con->write_atomic)
 			return false;
 	} else {
 		if (!con->write)
@@ -149,7 +150,8 @@ 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; }
+static inline bool console_is_usable(struct console *con, short flags,
+				     bool use_atomic) { return false; }
 
 #endif /* CONFIG_PRINTK */
 
diff --git a/kernel/printk/nbcon.c b/kernel/printk/nbcon.c
index 917022105b8b..ae9cd6d0b740 100644
--- a/kernel/printk/nbcon.c
+++ b/kernel/printk/nbcon.c
@@ -914,6 +914,7 @@ EXPORT_SYMBOL_GPL(nbcon_reacquire_nobuf);
 /**
  * nbcon_emit_next_record - Emit a record in the acquired context
  * @wctxt:	The write context that will be handed to the write function
+ * @use_atomic:	True if the write_atomic() callback is to be used
  *
  * Return:	True if this context still owns the console. False if
  *		ownership was handed over or taken.
@@ -927,7 +928,7 @@ EXPORT_SYMBOL_GPL(nbcon_reacquire_nobuf);
  * When true is returned, @wctxt->ctxt.backlog indicates whether there are
  * still records pending in the ringbuffer,
  */
-static bool nbcon_emit_next_record(struct nbcon_write_context *wctxt)
+static bool nbcon_emit_next_record(struct nbcon_write_context *wctxt, bool use_atomic)
 {
 	struct nbcon_context *ctxt = &ACCESS_PRIVATE(wctxt, ctxt);
 	struct console *con = ctxt->console;
@@ -938,6 +939,18 @@ static bool nbcon_emit_next_record(struct nbcon_write_context *wctxt)
 	unsigned long con_dropped;
 	unsigned long dropped;
 
+	/*
+	 * This function should never be called for consoles that have not
+	 * implemented the necessary callback for writing: i.e. legacy
+	 * consoles and, when atomic, nbcon consoles with no write_atomic().
+	 * Handle it as if ownership was lost and try to continue.
+	 */
+	if (WARN_ON_ONCE((use_atomic && !con->write_atomic) ||
+			 !(console_srcu_read_flags(con) & CON_NBCON))) {
+		nbcon_context_release(ctxt);
+		return false;
+	}
+
 	/*
 	 * The printk buffers are filled within an unsafe section. This
 	 * prevents NBCON_PRIO_NORMAL and NBCON_PRIO_EMERGENCY from
@@ -972,7 +985,7 @@ static bool nbcon_emit_next_record(struct nbcon_write_context *wctxt)
 	/* Initialize the write context for driver callbacks. */
 	nbcon_write_context_set_buf(wctxt, &pmsg.pbufs->outbuf[0], pmsg.outbuf_len);
 
-	if (con->write_atomic) {
+	if (use_atomic) {
 		con->write_atomic(con, wctxt);
 	} else {
 		/*
@@ -1104,7 +1117,7 @@ static bool nbcon_atomic_emit_one(struct nbcon_write_context *wctxt)
 	 * The higher priority printing context takes over responsibility
 	 * to print the pending records.
 	 */
-	if (!nbcon_emit_next_record(wctxt))
+	if (!nbcon_emit_next_record(wctxt, true))
 		return false;
 
 	nbcon_context_release(ctxt);
@@ -1205,7 +1218,7 @@ static int __nbcon_atomic_flush_pending_con(struct console *con, u64 stop_seq,
 		 * handed over or taken over. In both cases the context is no
 		 * longer valid.
 		 */
-		if (!nbcon_emit_next_record(&wctxt))
+		if (!nbcon_emit_next_record(&wctxt, true))
 			return -EAGAIN;
 
 		if (!ctxt->backlog) {
@@ -1294,7 +1307,7 @@ static void __nbcon_atomic_flush_pending(u64 stop_seq, bool allow_unsafe_takeove
 		if (!(flags & CON_NBCON))
 			continue;
 
-		if (!console_is_usable(con, flags))
+		if (!console_is_usable(con, flags, true))
 			continue;
 
 		if (nbcon_seq_read(con) >= stop_seq)
@@ -1552,7 +1565,7 @@ void nbcon_device_release(struct console *con)
 	printk_get_console_flush_type(&ft, false);
 	cookie = console_srcu_read_lock();
 	if (ft.nbcon_atomic &&
-	    console_is_usable(con, console_srcu_read_flags(con)) &&
+	    console_is_usable(con, console_srcu_read_flags(con), true) &&
 	    prb_read_valid(prb, nbcon_seq_read(con), NULL)) {
 		__nbcon_atomic_flush_pending_con(con, prb_next_reserve_seq(prb), false);
 	}
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 3fbe27551f75..e5e575384b3c 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -3018,7 +3018,7 @@ static bool console_flush_all(bool do_cond_resched, u64 *next_seq, bool *handove
 			u64 printk_seq;
 			bool progress;
 
-			if (!console_is_usable(con, flags))
+			if (!console_is_usable(con, flags, true))
 				continue;
 			any_usable = true;
 
@@ -3717,7 +3717,7 @@ static int unregister_console_locked(struct console *console)
 
 	if (!console_is_registered_locked(console))
 		res = -ENODEV;
-	else if (console_is_usable(console, console->flags))
+	else if (console_is_usable(console, console->flags, true))
 		__pr_flush(console, 1000, true);
 
 	/* Disable it unconditionally */
@@ -3993,7 +3993,7 @@ static bool __pr_flush(struct console *con, int timeout_ms, bool reset_on_progre
 			 * that they make forward progress, so only increment
 			 * @diff for usable consoles.
 			 */
-			if (!console_is_usable(c, flags))
+			if (!console_is_usable(c, flags, true))
 				continue;
 
 			if (flags & CON_NBCON) {
-- 
2.39.2