printk: add functions to prefer direct printing
Once kthread printing is available, console printing will no longer occur in the context of the printk caller. However, there are some special contexts where it is desirable for the printk caller to directly print out kernel messages. Using pr_flush() to wait for threaded printers is only possible if the caller is in a sleepable context and the kthreads are active. That is not always the case. Introduce printk_prefer_direct_enter() and printk_prefer_direct_exit() functions to explicitly (and globally) activate/deactivate preferred direct console printing. The term "direct console printing" refers to printing to all enabled consoles from the context of the printk caller. The term "prefer" is used because this type of printing is only best effort. If the console is currently locked or other printers are already actively printing, the printk caller will need to rely on the other contexts to handle the printing. This preferred direct printing is how all printing has been handled until now (unless it was explicitly deferred). When kthread printing is introduced, there may be some unanticipated problems due to kthreads being unable to flush important messages. In order to minimize such risks, preferred direct printing is activated for the primary important messages when the system experiences general types of major errors. These are: - emergency reboot/shutdown - cpu and rcu stalls - hard and soft lockups - hung tasks - warn - sysrq Note that since kthread printing does not yet exist, no behavior changes result from this commit. This is only implementing the counter and marking the various places where preferred direct printing is active. Signed-off-by:John Ogness <john.ogness@linutronix.de> Reviewed-by:
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