On 2019-02-13, Sergey Senozhatsky <sergey.senozhatsky.work@xxxxxxxxx> wrote: >> - console_flush_on_panic() currently is a NOP. It is pretty clear how >> this could be implemented if atomic_write was available. But if no >> such console is registered, it is not clear what should be done. Is >> this function really even needed? > > If you now rely on a fully preemptible printk kthread to flush > pending logbuf messages, then console_flush_on_panic() is your > only chance to see those pending logbuf messages on the serial > console when the system dies. Anything critical would have already been immediately print to the emergency consoles. And if an emergency console was available, console_flush_on_panic() could be a special case where _all_ unseen messages (regardless of importance) are printed to the emergency console. > Non-atomic consoles should become atomic once you call bust_spinlocks(1), > this is what we currently have: > > panic() > bust_spinlocks(1) // sets oops_in_progress > console_flush_on_panic() > call_console_drivers() > -> serial_driver_write() > if (oops_in_progress) > locked = spin_trylock_irqsave(&port->lock); > uart_console_write(); > if (locked) > spin_unlock_irqrestore(&port->lock); I don't like bust_spinlocks() because drivers end up implementing oops_in_progress with exactly that... ignoring their own locks. I prefer consoles are provided with a locking mechanism that they can use to support a separate NMI-safe write function. My series introduces console_atomic_lock() for exactly this purpose. But this doesn't help here. Here we are talking about a crashing system that does _not_ have an emergency console. And in this case I would say messages would be lost (just like they are now if all you have is a vt console and it was busy). I suppose we could keep the current bust_spinlocks() stuff for the special case that there are no emergency consoles available. It's better than nothing, but also not really reliable. Preferrably we figure out how to implement write_atomic for all console drivers. John Ogness
