Re: [RFC][PATCHv3 0/6] printk: use printk_safe to handle printk() recursive calls

Peter Zijlstra
2016-10-18T17:09:18Z

On Wed, Oct 19, 2016 at 12:40:39AM +0900, Sergey Senozhatsky wrote:
> Hello,
>
> RFC
>
> This patch set extends a lock-less NMI per-cpu buffers idea to
> handle recursive printk() calls. The basic mechanism is pretty much the
> same -- at the beginning of a deadlock-prone section we switch to lock-less
> printk callback, and return back to a default printk implementation at the
> end; the messages are getting flushed to a logbuf buffer from a safer
> context.

So I think you're not taking this far enough. You've also missed an
entire class of deadlocks.

The first is that you still keep the logbuf. Having this global
serialized thing is a source of fail. It would be much better to only
keep per cpu stuff. _OR_ at the very least make the logbuf itself
lockfree. So generate the printk entry local (so we know its size) then
atomically reserve the logbuf entry and copy it over.

The entire class of deadlocks you've missed is that console->write() is
a piece of crap too ;-) Even the bog standard 8250 serial console driver
can do wakeups.

See for example:

https://marc.info/?l=linux-kernel&m=132446649823347&w=2

I've entirely given up on printk(), I'll post the 3 patches I carry to
make things work for me.