From: Ingo Molnar <mingo@elte.hu>

printk currently does

	if (oops_in_progres)
		bust_printk_locks();

which means that once we oops, the printk locking is 100% ineffective and
multiple CPUs make an unreadable mess on a serial console.  It's a significant
development hassle.

Fix that up by only popping locks once per ten seconds.

akpm@osdl.org did:

  - Bump the timeout to 30 seconds - 9600 baud is slow.

  - Handle jiffy wraps: change the logic so that we only skip the lockbust
    if the current time is within 30 seconds of the previous lockbusting
    attempt.