printk is supposed to be callable from anywhere, but various people have noted
that that's not really the case.  See, for example, bugs 1594, 1837, 1564, and
1776.  This bug report has a little more in the way of specifics on that topic.

In 1776, calling printk from a handler for a probe in activate_task yielded
oopses and/or hangs.  In that particular case, I think a deadlock is caused as
follows: activate_task -> handler -> printk -> release_console_sem [locks
logbuf_lock] -> up -> __up_wakeup -> __up -> wake_up -> __wake_up ->
__wake_up_common -> default_wake_function -> try_to_wake_up -> activate_task ->
handler -> printk [goes for logbuf_lock again].

This is not a problem for SystemTap, which doesn't emit printk calls, but it
could be for kprobes users.  Possible actions we could take include:

1) Document this type of scenario in Documentation/kprobes.txt.  I.e., a handler
for any function callable while logbuf_lock is held shouldn't call printk.

2) Push for an enhancement to printk.c -- e.g.,
a) In release_console_sem, consider releasing logbuf_lock and console_sem in
reverse order.  Unfortunately, while logbuf_lock is held there's also a call to
down_trylock, which can indirectly call wake_up_locked, so that doesn't buy us much.
b) Upon entry to printk, consider calling spin_trylock instead of spin_lock, and
print nothing (and maybe atomic_inc a miss-counter to report later) if we can't
get logbuf_lock.