[PATCH v2] printk: Skip console drivers on PREEMPT_RT.

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

printk might be invoked in a context with disabled interrupts and or
preemption and additionally disables interrupts before it invokes the
console drivers. This is behaviour is not compatible with PREEMPT_RT.

Disable console printing until the return of atomic consoles and the
printing thread. This allows to retrieve the log buffer from user space
which is not possible by disable printk.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
v1…v2:
   - Use __console_unlock() as suggested by John.

 kernel/printk/printk.c |   10 ++++++++++
 1 file changed, 10 insertions(+)

--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2843,6 +2843,16 @@ void console_unlock(void)
 	}
 
 	/*
+	 * On PREEMPT_RT it is not possible to invoke console drivers with
+	 * disabled interrupts and or preemption. Therefore all drivers are
+	 * skipped and the output can be retrieved from the buffer.
+	 */
+	if (IS_ENABLED(CONFIG_PREEMPT_RT)) {
+		__console_unlock();
+		return;
+	}
+
+	/*
 	 * Console drivers are called with interrupts disabled, so
 	 * @console_may_schedule should be cleared before; however, we may
 	 * end up dumping a lot of lines, for example, if called from