[PATCH printk v2 04/11] printk: nbcon: Provide functions to mark atomic write sections

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

From: Thomas Gleixner <tglx@linutronix.de>

WARN/OOPS/PANIC require printing out immediately since the
regular printing method (and in the future, the printing
threads) might not be able to run.

Add per-CPU state to denote the priority/urgency of the output
and provide functions to mark the beginning and end of sections
where the urgent messages are generated.

Note that when a CPU is in a priority elevated state, flushing
only occurs when dropping back to a lower priority. This allows
the full set of printk records (WARN/OOPS/PANIC output) to be
stored in the ringbuffer before beginning to flush the backlog.

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/nbcon.c   | 89 +++++++++++++++++++++++++++++++++++++++++
 2 files changed, 93 insertions(+)

diff --git a/include/linux/console.h b/include/linux/console.h
index e4fc6f7c1496..25a3ddd39083 100644
--- a/include/linux/console.h
+++ b/include/linux/console.h
@@ -452,10 +452,14 @@ static inline bool console_is_registered(const struct console *con)
 	hlist_for_each_entry(con, &console_list, node)
 
 #ifdef CONFIG_PRINTK
+extern enum nbcon_prio nbcon_atomic_enter(enum nbcon_prio prio);
+extern void nbcon_atomic_exit(enum nbcon_prio prio, enum nbcon_prio prev_prio);
 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 enum nbcon_prio nbcon_atomic_enter(enum nbcon_prio prio) { return NBCON_PRIO_NONE; }
+static inline void nbcon_atomic_exit(enum nbcon_prio prio, enum nbcon_prio prev_prio) { }
 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; }
diff --git a/kernel/printk/nbcon.c b/kernel/printk/nbcon.c
index b96077152f49..9359906b575b 100644
--- a/kernel/printk/nbcon.c
+++ b/kernel/printk/nbcon.c
@@ -961,6 +961,95 @@ static bool nbcon_emit_next_record(struct nbcon_write_context *wctxt)
 	return nbcon_context_exit_unsafe(ctxt);
 }
 
+/**
+ * struct nbcon_cpu_state - Per CPU printk context state
+ * @prio:	The current context priority level
+ * @nesting:	Per priority nest counter
+ */
+struct nbcon_cpu_state {
+	enum nbcon_prio		prio;
+	int			nesting[NBCON_PRIO_MAX];
+};
+
+static DEFINE_PER_CPU(struct nbcon_cpu_state, nbcon_pcpu_state);
+static struct nbcon_cpu_state early_nbcon_pcpu_state __initdata;
+
+/**
+ * nbcon_get_cpu_state - Get the per CPU console state pointer
+ *
+ * Returns either a pointer to the per CPU state of the current CPU or to
+ * the init data state during early boot.
+ */
+static __ref struct nbcon_cpu_state *nbcon_get_cpu_state(void)
+{
+	if (!printk_percpu_data_ready())
+		return &early_nbcon_pcpu_state;
+
+	return this_cpu_ptr(&nbcon_pcpu_state);
+}
+
+/**
+ * nbcon_atomic_enter - Enter a context that enforces atomic printing
+ * @prio:	Priority of the context
+ *
+ * Return:	The previous priority that needs to be fed into
+ *		the corresponding nbcon_atomic_exit()
+ * Context:	Any context. Disables migration.
+ */
+enum nbcon_prio nbcon_atomic_enter(enum nbcon_prio prio)
+{
+	struct nbcon_cpu_state *cpu_state;
+	enum nbcon_prio prev_prio;
+
+	migrate_disable();
+
+	cpu_state = nbcon_get_cpu_state();
+
+	prev_prio = cpu_state->prio;
+	if (prio > prev_prio)
+		cpu_state->prio = prio;
+
+	/*
+	 * Increment the nesting on @cpu_state->prio (instead of
+	 * @prio) so that a WARN() nested within a panic printout
+	 * does not attempt to scribble state.
+	 */
+	cpu_state->nesting[cpu_state->prio]++;
+
+	return prev_prio;
+}
+
+/**
+ * nbcon_atomic_exit - Exit a context that enforces atomic printing
+ * @prio:	Priority of the context to leave
+ * @prev_prio:	Priority of the previous context for restore
+ *
+ * Context:	Any context. Enables migration.
+ *
+ * @prev_prio is the priority returned by the corresponding
+ * nbcon_atomic_enter().
+ */
+void nbcon_atomic_exit(enum nbcon_prio prio, enum nbcon_prio prev_prio)
+{
+	struct nbcon_cpu_state *cpu_state;
+
+	cpu_state = nbcon_get_cpu_state();
+
+	/*
+	 * Undo the nesting of nbcon_atomic_enter() at the CPU state
+	 * priority.
+	 */
+	cpu_state->nesting[cpu_state->prio]--;
+
+	/*
+	 * Restore the previous priority, which was returned by
+	 * nbcon_atomic_enter().
+	 */
+	cpu_state->prio = prev_prio;
+
+	migrate_enable();
+}
+
 /**
  * nbcon_alloc - Allocate buffers needed by the nbcon console
  * @con:	Console to allocate buffers for
-- 
2.39.2