On Tue 2019-02-12 15:29:58, John Ogness wrote: > Implement a non-sleeping NMI-safe write_atomic console function in > order to support emergency printk messages. It uses console_atomic_lock() added in 18th patch. That one uses prb_lock() added by 2nd patch. Now, prb_lock() allows recursion on the same CPU. But it still needs to wait until it is released on another CPU. It means that it is not completely save when NMIs happen on more CPUs in parallel, for example, when calling nmi_trigger_cpumask_backtrace(). OK, it would be safe when prb_lock() is the only lock taken in the NMI handler. But printk() should not make such limitation to the rest of the system. Not to say, that we would most likely need to add a lock back into nmi_cpu_backtrace() to keep the output sane. Peter Zijlstra several times talked about fully lockless consoles. He is using the early console for debugging, see the patchset https://lkml.kernel.org/r/20170928121823.430053219@xxxxxxxxxxxxx I am not sure if it is always possible. I personally see the following way: 1. Make the printk ring buffer fully lockless. Then we reduce the problem only to console locking. And we could have a per-console-driver lock (no the big lock like prb_lock()). 2. I am afraid that we need to add some locking between CPUs to avoid mixing characters from directly printed messages. This would be safe everywhere expect in NMI. Then we could either risk ignoring the lock in NMI (there should be few messages anyway, the backtraces would get synchronized another way). Or we might need a compromise between handling console using the current owner and offload. Best Regards, Petr
