Provide a deferred version of the WARN_ON() macro. It will delay
flushing the console until a later context. It is needed in a context
where the caller holds locks which can lead to a deadlock content is
flushed to the console driver.
An example would from a warning from within the scheduler resulting in a
wake-up of a task.

Deferring the output works by using printk_deferred_enter/ exit() around
the printing output. This must be used in a context where the task can't
migrate to another CPU. This should be the case usually, since the
scheduler would acquire the rq lock whith disabled interrupts, but to be
safe preemption is disabled to guarantee this.

In order not to bloat the code on architectures which provide an
optimized __WARN_FLAGS() define BUGFLAG_DEFERRED which is handled by
__report_bug() and does not increase the code size.

Provide the DEFERRED macros based on __WARN_FLAGS and __WARN_FLAGS
macros. Extend __report_bug() to handle the deferred case.

...

--- a/include/asm-generic/bug.h
+++ b/include/asm-generic/bug.h

@@ -229,7 +230,10 @@ static enum bug_trap_type __report_bug(struct bug_entry *bug, unsigned long buga
 		 */
 		bug->flags |= BUGFLAG_DONE;
 	}
-
+	if (deferred) {
+		preempt_disable_notrace();
+		printk_deferred_enter();
+	}