Re: [RFC][PATCH 0/7] printk: use alt_printk to handle printk() recursive calls

[Sergey Senozhatsky]

On (09/29/16 15:25), Petr Mladek wrote:
> On Tue 2016-09-27 23:22:30, 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.
> 
> I was skeptical but I really like this way now.
>
> The switching of the buffers is a bit hairy in this version but I
> think that we could make it much better.
> 
> Other than that it looks like a big win. It kills a lot of
> printk-related pain points. And it will not be that complicated
> after all.

many thanks for looking at this train wreck.

so, like I said, it addresses printk()-recursion in *ideally* quite
a minimalistic way -- just several alt_printk_enter/exit calls in
printk.c, without ever touching any other parts of the kernel.

gunning down printk deadlocks in general, however, requires much more
effort; or even a completely different approach.

a) a lock-less printk() by default
   um, `#define printk alt_printk'. but this will break printk() from irq.
   and the ordering of messages from per-cpu buffers may be far from correct.

b) combining a DEFERRED_WARN + alt_printk
   DEFERRED_WARN potentially is a never ending thing. we can add some
   lockdep annotations, perhaps, and hope that error handling branches
   that may contain WARN_ONs/printk-s will be executed with prove_locking
   enabled on someone's machine.

c) ...

	-ss