[PATCH printk v2 8/8] printk: nbcon: Add functions for drivers to mark unsafe regions

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

From: Thomas Gleixner <tglx@linutronix.de>

For the write_atomic callback, the console driver may have unsafe
regions that need to be appropriately marked. Provide functions
that accept the nbcon_write_context struct to allow for the driver
to enter and exit unsafe regions.

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>
---
 include/linux/console.h      |  4 ++++
 kernel/printk/printk_nbcon.c | 41 +++++++++++++++++++++++++++++++++++-
 2 files changed, 44 insertions(+), 1 deletion(-)

diff --git a/include/linux/console.h b/include/linux/console.h
index 3d129b2b70a1..e4ce1015627c 100644
--- a/include/linux/console.h
+++ b/include/linux/console.h
@@ -456,8 +456,12 @@ static inline bool console_is_registered(const struct console *con)
 
 #ifdef CONFIG_PRINTK
 extern bool nbcon_can_proceed(struct nbcon_write_context *wctxt);
+extern bool nbcon_enter_unsafe(struct nbcon_write_context *wctxt);
+extern bool nbcon_exit_unsafe(struct nbcon_write_context *wctxt);
 #else
 static inline bool nbcon_can_proceed(struct nbcon_write_context *wctxt) { return false; }
+static inline bool nbcon_enter_unsafe(struct nbcon_write_context *wctxt) { return false; }
+static inline bool nbcon_exit_unsafe(struct nbcon_write_context *wctxt) { return false; }
 #endif
 
 extern int console_set_on_cmdline;
diff --git a/kernel/printk/printk_nbcon.c b/kernel/printk/printk_nbcon.c
index e1f0e4278ffa..57b7539bbbb2 100644
--- a/kernel/printk/printk_nbcon.c
+++ b/kernel/printk/printk_nbcon.c
@@ -761,7 +761,6 @@ EXPORT_SYMBOL_GPL(nbcon_can_proceed);
  *
  * Internal helper to avoid duplicated code
  */
-__maybe_unused
 static bool nbcon_context_update_unsafe(struct nbcon_context *ctxt, bool unsafe)
 {
 	struct console *con = ctxt->console;
@@ -787,6 +786,46 @@ static bool nbcon_context_update_unsafe(struct nbcon_context *ctxt, bool unsafe)
 	return true;
 }
 
+/**
+ * nbcon_enter_unsafe - Enter an unsafe region in the driver
+ * @wctxt:	The write context that was handed to the write function
+ *
+ * Return:	True if the state is correct. False if ownership was
+ *		handed over or taken.
+ *
+ * When this function returns false then the calling context is no longer
+ * the owner and is no longer allowed to go forward. In this case it must
+ * back out immediately and carefully. The buffer content is also no longer
+ * trusted since it no longer belongs to the calling context.
+ */
+bool nbcon_enter_unsafe(struct nbcon_write_context *wctxt)
+{
+	struct nbcon_context *ctxt = &ACCESS_PRIVATE(wctxt, ctxt);
+
+	return nbcon_context_update_unsafe(ctxt, true);
+}
+EXPORT_SYMBOL_GPL(nbcon_enter_unsafe);
+
+/**
+ * nbcon_exit_unsafe - Exit an unsafe region in the driver
+ * @wctxt:	The write context that was handed to the write function
+ *
+ * Return:	True if the state is correct. False if ownership was
+ *		handed over or taken.
+ *
+ * When this function returns false then the calling context is no longer
+ * the owner and is no longer allowed to go forward. In this case it must
+ * back out immediately and carefully. The buffer content is also no longer
+ * trusted since it no longer belongs to the calling context.
+ */
+bool nbcon_exit_unsafe(struct nbcon_write_context *wctxt)
+{
+	struct nbcon_context *ctxt = &ACCESS_PRIVATE(wctxt, ctxt);
+
+	return nbcon_context_update_unsafe(ctxt, false);
+}
+EXPORT_SYMBOL_GPL(nbcon_exit_unsafe);
+
 /**
  * nbcon_emit_next_record - Emit a record in the acquired context
  * @wctxt:	The write context that will be handed to the write function
-- 
2.39.2