[PATCH v3 2/4] printk: nbcon: Introduce KDB helpers

[Marcos Paulo de Souza <mpdesouza@suse.com>]

These helpers will be used when calling console->write_atomic on
KDB code in the next patch. It's basically the same implementaion
as nbcon_device_try_acquire, but using NBCON_PORIO_EMERGENCY when
acquiring the context.

For release we need to flush the console, since some messages could be
added before the context was acquired, as KDB emits the messages using
con->{write,write_atomic} instead of storing them on the ring buffer.

Signed-off-by: Marcos Paulo de Souza <mpdesouza@suse.com>
---
 include/linux/console.h |  6 +++++
 kernel/printk/nbcon.c   | 66 +++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 72 insertions(+)

diff --git a/include/linux/console.h b/include/linux/console.h
index 67af483574727c00eea1d5a1eacc994755c92607..b34c5a0b86303e2fb4583fa467d8be43761cf756 100644
--- a/include/linux/console.h
+++ b/include/linux/console.h
@@ -605,6 +605,9 @@ 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);
 extern void nbcon_reacquire_nobuf(struct nbcon_write_context *wctxt);
+extern bool nbcon_kdb_try_acquire(struct console *con,
+				  struct nbcon_write_context *wctxt);
+extern void nbcon_kdb_release(struct nbcon_write_context *wctxt);
 
 /*
  * Check if the given console is currently capable and allowed to print
@@ -654,6 +657,9 @@ static inline bool nbcon_can_proceed(struct nbcon_write_context *wctxt) { return
 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; }
 static inline void nbcon_reacquire_nobuf(struct nbcon_write_context *wctxt) { }
+static inline bool nbcon_kdb_try_acquire(struct console *con,
+					 struct nbcon_write_context *wctxt) { return false; }
+static inline void nbcon_kdb_release(struct console *con) { }
 static inline bool console_is_usable(struct console *con, short flags,
 				     bool use_atomic) { return false; }
 #endif
diff --git a/kernel/printk/nbcon.c b/kernel/printk/nbcon.c
index 646801813415f0abe40cabf2f28ca9e30664f028..ff218e95a505fd10521c2c4dfb00ad5ec5773953 100644
--- a/kernel/printk/nbcon.c
+++ b/kernel/printk/nbcon.c
@@ -1855,3 +1855,69 @@ void nbcon_device_release(struct console *con)
 	console_srcu_read_unlock(cookie);
 }
 EXPORT_SYMBOL_GPL(nbcon_device_release);
+
+/**
+ * nbcon_kdb_try_acquire - Try to acquire nbcon console, enter unsafe
+ *				section, and initialized nbcon write context
+ * @con:	The nbcon console to acquire
+ * @wctxt:	The nbcon write context to be used on success
+ *
+ * Context:	Under console_srcu_read_lock() for emiting a single kdb message
+ *		using the given con->write_atomic() callback. Can be called
+ *		only when the console is usable at the moment.
+ *
+ * Return:	True if the console was acquired. False otherwise.
+ *
+ * kdb emits messages on consoles registered for printk() without
+ * storing them into the ring buffer. It has to acquire the console
+ * ownerhip so that it could call con->write_atomic() callback a safe way.
+ *
+ * This function acquires the nbcon console using priority NBCON_PRIO_EMERGENCY
+ * and marks it unsafe for handover/takeover.
+ */
+bool nbcon_kdb_try_acquire(struct console *con,
+			   struct nbcon_write_context *wctxt)
+{
+	struct nbcon_context *ctxt = &ACCESS_PRIVATE(wctxt, ctxt);
+
+	memset(ctxt, 0, sizeof(*ctxt));
+	ctxt->console = con;
+	ctxt->prio    = NBCON_PRIO_EMERGENCY;
+
+	if (!nbcon_context_try_acquire(ctxt, false))
+		return false;
+
+	if (!nbcon_context_enter_unsafe(ctxt))
+		return false;
+
+	return true;
+}
+
+/**
+ * nbcon_kdb_release - Exit unsafe section and release the nbcon console
+ *
+ * @wctxt:	The nbcon write context initialized by a successful
+ *		nbcon_kdb_try_acquire()
+ *
+ * Context:	Under console_srcu_read_lock() for emiting a single kdb message
+ *		using the given con->write_atomic() callback. Can be called
+ *		only when the console is usable at the moment.
+ */
+void nbcon_kdb_release(struct nbcon_write_context *wctxt)
+{
+	struct nbcon_context *ctxt = &ACCESS_PRIVATE(wctxt, ctxt);
+
+	if (!nbcon_context_exit_unsafe(ctxt))
+		return;
+
+	nbcon_context_release(ctxt);
+
+	/*
+	 * Flush any new printk() messages added when the console was blocked.
+	 * Only the console used by the given write context was	blocked.
+	 * The console was locked only when the write_atomic() callback
+	 * was usable.
+	 */
+	__nbcon_atomic_flush_pending_con(ctxt->console,
+					 prb_next_reserve_seq(prb), false);
+}

-- 
2.50.0