nbcon: Add API to acquire context for non-printing operations

Provide functions nbcon_device_try_acquire() and
nbcon_device_release() which will try to acquire the nbcon
console ownership with NBCON_PRIO_NORMAL and mark it unsafe for
handover/takeover.

These functions are to be used together with the device-specific
locking when performing non-printing activities on the console
device. They will allow synchronization against the
atomic_write() callback which will be serialized, for higher
priority contexts, only by acquiring the console context
ownership.

Pitfalls:

The API requires to be called in a context with migration
disabled because it uses per-CPU variables internally.

The context is set unsafe for a takeover all the time. It
guarantees full serialization against any atomic_write() caller
except for the final flush in panic() which might try an unsafe
takeover.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20240820063001.36405-14-john.ogness@linutronix.de
Signed-off-by: Petr Mladek <pmladek@suse.com>

2 files changed, 16 insertions, 0 deletions

diff --git a/include/linux/console.h b/include/linux/console.h
index aafe3121b74e..3706f944de46 100644
--- a/include/linux/console.h
+++ b/include/linux/console.h
@@ -322,6 +322,7 @@ struct nbcon_write_context {
  *
  * @nbcon_state:	State for nbcon consoles
  * @nbcon_seq:		Sequence number of the next record for nbcon to print
+ * @nbcon_device_ctxt:	Context available for non-printing operations
  * @pbufs:		Pointer to nbcon private buffer
  */
 struct console {
@@ -417,6 +418,7 @@ struct console {
 
 	atomic_t		__private nbcon_state;
 	atomic_long_t		__private nbcon_seq;
+	struct nbcon_context	__private nbcon_device_ctxt;
 	struct printk_buffers	*pbufs;
 };
 
diff --git a/include/linux/printk.h b/include/linux/printk.h
index eee8e97da681..9687089f5ace 100644
--- a/include/linux/printk.h
+++ b/include/linux/printk.h
@@ -9,6 +9,8 @@
 #include <linux/ratelimit_types.h>
 #include <linux/once_lite.h>
 
+struct console;
+
 extern const char linux_banner[];
 extern const char linux_proc_banner[];
 
@@ -198,6 +200,8 @@ extern asmlinkage void dump_stack_lvl(const char *log_lvl) __cold;
 extern asmlinkage void dump_stack(void) __cold;
 void printk_trigger_flush(void);
 void console_try_replay_all(void);
+extern bool nbcon_device_try_acquire(struct console *con);
+extern void nbcon_device_release(struct console *con);
 #else
 static inline __printf(1, 0)
 int vprintk(const char *s, va_list args)
@@ -280,6 +284,16 @@ static inline void printk_trigger_flush(void)
 static inline void console_try_replay_all(void)
 {
 }
+
+static inline bool nbcon_device_try_acquire(struct console *con)
+{
+	return false;
+}
+
+static inline void nbcon_device_release(struct console *con)
+{
+}
+
 #endif
 
 bool this_cpu_in_panic(void);