[PATCH printk v2 6/8] printk: nbcon: Add ownership state functions

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

From: Thomas Gleixner <tglx@linutronix.de>

Provide functions that are related to the safe handover mechanism
and allow console drivers to dynamically specify unsafe regions:

 - nbcon_context_can_proceed()

   Invoked by a console owner to check whether a handover request
   is pending or whether the console was taken over in a hostile
   fashion. If a handover request is pending, this function will
   also perform the handover, thus cancelling its own ownership.

 - nbcon_context_update_unsafe()

   Invoked by a console owner to denote that the driver is about
   to enter or leave a critical region where a hostile take over
   is unsafe. This function is also a cancellation point where
   loss of ownership can occur.

   The unsafe state is stored in the console state and allows a
   new context to make informed decisions whether to attempt a
   takeover of such a console. The unsafe state is also available
   to the driver so that it can make informed decisions about the
   required actions or take a special emergency path.

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/printk_nbcon.c | 113 ++++++++++++++++++++++++++++++++++-
 1 file changed, 112 insertions(+), 1 deletion(-)

diff --git a/kernel/printk/printk_nbcon.c b/kernel/printk/printk_nbcon.c
index 8229a0a00d5b..e41f2eff5ef6 100644
--- a/kernel/printk/printk_nbcon.c
+++ b/kernel/printk/printk_nbcon.c
@@ -623,7 +623,6 @@ static bool nbcon_owner_matches(struct nbcon_state *cur, int expected_cpu,
  * nbcon_context_release - Release the console
  * @ctxt:	The nbcon context from nbcon_context_try_acquire()
  */
-__maybe_unused
 static void nbcon_context_release(struct nbcon_context *ctxt)
 {
 	unsigned int cpu = smp_processor_id();
@@ -650,6 +649,118 @@ static void nbcon_context_release(struct nbcon_context *ctxt)
 	ctxt->pbufs = NULL;
 }
 
+/**
+ * nbcon_context_can_proceed - Check whether ownership can proceed
+ * @ctxt:	The nbcon context from nbcon_context_try_acquire()
+ * @cur:	The current console state
+ *
+ * Return:	True if the state is correct. False if ownership was
+ *		handed over or taken.
+ *
+ * Must be invoked after the record was dumped into the assigned buffer
+ * and at appropriate safe places in the driver.
+ *
+ * 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.
+ */
+static bool nbcon_context_can_proceed(struct nbcon_context *ctxt, struct nbcon_state *cur)
+{
+	unsigned int cpu = smp_processor_id();
+
+	/* Make sure this context is still the owner. */
+	if (!nbcon_owner_matches(cur, cpu, ctxt->prio)) {
+		debug_store(1, "handover: cpu%d DETECTED HOSTILE takeover\n", cpu);
+		return false;
+	}
+
+	/* The console owner can proceed if there is no waiter. */
+	if (cur->req_prio == NBCON_PRIO_NONE)
+		return true;
+
+	/*
+	 * A console owner within an unsafe region is always allowed to
+	 * proceed, even if there are waiters. It can perform a handover
+	 * when exiting the unsafe region. Otherwise the waiter will
+	 * need to perform an unsafe hostile takeover.
+	 */
+	if (cur->unsafe) {
+		debug_store(cur->req_prio > cur->prio,
+			    "handover: cpu%d IGNORING HANDOVER prio%d -> prio%d (unsafe)\n",
+			    cpu, cur->prio, cur->req_prio);
+		return true;
+	}
+
+	/* Waiters always have higher priorities than owners. */
+	WARN_ON_ONCE(cur->req_prio <= cur->prio);
+
+	debug_store(1, "handover: cpu%d HANDING OVER prio%d -> prio%d\n",
+		    cpu, cur->prio, cur->req_prio);
+
+
+	/*
+	 * Having a safe point for take over and eventually a few
+	 * duplicated characters or a full line is way better than a
+	 * hostile takeover. Post processing can take care of the garbage.
+	 * Release and hand over.
+	 */
+	nbcon_context_release(ctxt);
+
+	/*
+	 * It is not known whether the handover succeeded. The outermost
+	 * callsite has to make the final decision whether printing
+	 * should proceed or not (via reacquire, possibly hostile). The
+	 * console is now unlocked so go back all the way instead of
+	 * trying to implement heuristics in tons of places.
+	 */
+	return false;
+}
+
+/**
+ * nbcon_context_update_unsafe - Update the unsafe bit in @con->nbcon_state
+ * @ctxt:	The nbcon context from nbcon_context_try_acquire()
+ * @unsafe:	The new value for the unsafe bit
+ *
+ * Return:	True if the state is correct. False if ownership was
+ *		handed over or taken.
+ *
+ * Typically the unsafe bit is set during acquire. This function allows
+ * modifying the unsafe status without releasing ownership.
+ *
+ * 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.
+ *
+ * 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;
+	struct nbcon_state cur;
+	struct nbcon_state new;
+
+	nbcon_state_read(con, &cur);
+
+	/* The unsafe bit must not be cleared if @hostile_unsafe is set. */
+	if (!unsafe && cur.hostile_unsafe)
+		return nbcon_context_can_proceed(ctxt, &cur);
+
+	do {
+		if (!nbcon_context_can_proceed(ctxt, &cur))
+			return false;
+
+		new.atom = cur.atom;
+		new.unsafe = unsafe;
+	} while (!nbcon_state_try_cmpxchg(con, &cur, &new));
+
+	ctxt->unsafe = unsafe;
+
+	return true;
+}
+
 /**
  * nbcon_init - Initialize the nbcon console specific data
  * @con:	Console to initialize
-- 
2.39.2