Re: [PATCH] printk: robustify printk

[Linus Torvalds]

On Fri, 8 Aug 2008, Andrew Morton wrote:
> 
> Why are we fixing this, btw?  The problem has been there forever and
> people who hack the wakeup code could/should know about it anyway.  All
> they need to do is to disable klogd during development.  Did the
> problem recently become worse for some reason?

It hasn't beemn there forever at all.

Yes, there used to be reliance on the actual _scheduler_ locks. Doign a 
wake_up() would cause runqueue locks etc to be taken.

But the xtime deadlock is fairly recent, and only happened with CFQ, I 
think.

And _that_ is the irritating one. I personally wouldn't mind at all if 
there is some printk() dependency on the core runqueue rq->lock or on the 
RCU locking thing. But look at xtime_lock. THAT is a disaster.

Just grep for it. 

So I personally actually like the RCU version best. Yes, it still depends 
on really core locking. But it's really core and low-level and _confined_ 
locking, where a comment in a single place would probably suffice. Compare 
that to all the places where we take the xtime_lock for writing!

		Linus
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