PREEMPT_RT (2.6.33-rt17) disabled printk-to-console after console_init

[Xianghua Xiao]

I would like to have printk output to my ttyS0 console all the time,
with 2.6.33-rt17, after kernel is booted up with all messages shown
normally on the serial port until I login in, from this point on, all
printk messages go to 'dmesg(log_buf)' fine, but I could never get
them shown up on serial console anymore. I tried adjust
/proc/sys/kernel/printk etc none helped.

Switch PREEMPT_RT to PREEMPT_VOLUNTARY and reboot, now everything
works, i.e. I can see printk on my console after I login.

Look at the RT patch itself, I do find some changes to printk itself,
however I'm unsure what is causing this(i.e printk not going to
console after login).

Thanks,
Xianghua
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