printk — print a kernel message
int printk (const char * fmt, ...);
This is printk. It can be called from any context. We want it to work.
We try to grab the console_sem. If we succeed, it's easy - we log the output and
call the console drivers. If we fail to get the semaphore we place the output
into the log buffer and return. The current holder of the console_sem will
notice the new output in release_console_sem and will send it to the
consoles before releasing the semaphore.
One effect of this deferred printing is that code which calls printk and
then changes console_loglevel may break. This is because console_loglevel
is inspected when the actual printing occurs.