1 files changed, 67 insertions, 7 deletions

diff --git a/kernel/printk/nbcon.c b/kernel/printk/nbcon.c
index cf62f675c673..8a1bf6f94bce 100644
--- a/kernel/printk/nbcon.c
+++ b/kernel/printk/nbcon.c
@@ -830,6 +830,19 @@ out:
 	return nbcon_context_can_proceed(ctxt, &cur);
 }
 
+static void nbcon_write_context_set_buf(struct nbcon_write_context *wctxt,
+					char *buf, unsigned int len)
+{
+	struct nbcon_context *ctxt = &ACCESS_PRIVATE(wctxt, ctxt);
+	struct console *con = ctxt->console;
+	struct nbcon_state cur;
+
+	wctxt->outbuf = buf;
+	wctxt->len = len;
+	nbcon_state_read(con, &cur);
+	wctxt->unsafe_takeover = cur.unsafe_takeover;
+}
+
 /**
  * nbcon_enter_unsafe - Enter an unsafe region in the driver
  * @wctxt:	The write context that was handed to the write function
@@ -845,8 +858,12 @@ out:
 bool nbcon_enter_unsafe(struct nbcon_write_context *wctxt)
 {
 	struct nbcon_context *ctxt = &ACCESS_PRIVATE(wctxt, ctxt);
+	bool is_owner;
 
-	return nbcon_context_enter_unsafe(ctxt);
+	is_owner = nbcon_context_enter_unsafe(ctxt);
+	if (!is_owner)
+		nbcon_write_context_set_buf(wctxt, NULL, 0);
+	return is_owner;
 }
 EXPORT_SYMBOL_GPL(nbcon_enter_unsafe);
 
@@ -865,12 +882,44 @@ EXPORT_SYMBOL_GPL(nbcon_enter_unsafe);
 bool nbcon_exit_unsafe(struct nbcon_write_context *wctxt)
 {
 	struct nbcon_context *ctxt = &ACCESS_PRIVATE(wctxt, ctxt);
+	bool ret;
 
-	return nbcon_context_exit_unsafe(ctxt);
+	ret = nbcon_context_exit_unsafe(ctxt);
+	if (!ret)
+		nbcon_write_context_set_buf(wctxt, NULL, 0);
+	return ret;
 }
 EXPORT_SYMBOL_GPL(nbcon_exit_unsafe);
 
 /**
+ * nbcon_reacquire_nobuf - Reacquire a console after losing ownership
+ *				while printing
+ * @wctxt:	The write context that was handed to the write callback
+ *
+ * Since ownership can be lost at any time due to handover or takeover, a
+ * printing context _must_ be prepared to back out immediately and
+ * carefully. However, there are scenarios where the printing context must
+ * reacquire ownership in order to finalize or revert hardware changes.
+ *
+ * This function allows a printing context to reacquire ownership using the
+ * same priority as its previous ownership.
+ *
+ * Note that after a successful reacquire the printing context will have no
+ * output buffer because that has been lost. This function cannot be used to
+ * resume printing.
+ */
+void nbcon_reacquire_nobuf(struct nbcon_write_context *wctxt)
+{
+	struct nbcon_context *ctxt = &ACCESS_PRIVATE(wctxt, ctxt);
+
+	while (!nbcon_context_try_acquire(ctxt))
+		cpu_relax();
+
+	nbcon_write_context_set_buf(wctxt, NULL, 0);
+}
+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
  *
@@ -895,7 +944,6 @@ static bool nbcon_emit_next_record(struct nbcon_write_context *wctxt)
 		.pbufs = ctxt->pbufs,
 	};
 	unsigned long con_dropped;
-	struct nbcon_state cur;
 	unsigned long dropped;
 
 	/*
@@ -930,10 +978,7 @@ static bool nbcon_emit_next_record(struct nbcon_write_context *wctxt)
 		goto update_con;
 
 	/* Initialize the write context for driver callbacks. */
-	wctxt->outbuf = &pmsg.pbufs->outbuf[0];
-	wctxt->len = pmsg.outbuf_len;
-	nbcon_state_read(con, &cur);
-	wctxt->unsafe_takeover = cur.unsafe_takeover;
+	nbcon_write_context_set_buf(wctxt, &pmsg.pbufs->outbuf[0], pmsg.outbuf_len);
 
 	if (con->write_atomic) {
 		con->write_atomic(con, wctxt);
@@ -947,6 +992,21 @@ static bool nbcon_emit_next_record(struct nbcon_write_context *wctxt)
 		return false;
 	}
 
+	if (!wctxt->outbuf) {
+		/*
+		 * Ownership was lost and reacquired by the driver. Handle it
+		 * as if ownership was lost.
+		 */
+		nbcon_context_release(ctxt);
+		return false;
+	}
+
+	/*
+	 * Ownership may have been lost but _not_ reacquired by the driver.
+	 * This case is detected and handled when entering unsafe to update
+	 * dropped/seq values.
+	 */
+
 	/*
 	 * Since any dropped message was successfully output, reset the
 	 * dropped count for the console.