linux-kernel - Re: [RFC][PATCH 6/7] printk: use alternative printk buffers

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Date:   Wed, 5 Oct 2016 10:27:14 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Jan Kara <jack@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Tejun Heo <tj@...nel.org>, Calvin Owens <calvinowens@...com>,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH 6/7] printk: use alternative printk buffers

On (10/04/16 16:52), Petr Mladek wrote:
> > 
> > Or is there any other catch that I do not see at the moment?
> 
> And there is :-( The above logic looked at the problem only from
> one side. It was about errors starting from the printk()
> code itself, for example:

yes, like I said - printk recursion and printk deadlock are different
things. and recursion cases are a subset of deadlock cases.

> The only thing that might help here is to call
> alt_printk_enter()/exit() in wake_up_process() itself. Otherwise,
> we still would need to keep the printk_deferred() stuff.

yes.
or
- combine alt_printk and DEFERRED_WARN/etc.
or
- rewrite printk() to be lock-less by default (for all invocations).

> By other words, we might need to put alt_printk_enter()/exit()
> into the scheduler and timekeeping code. In theory it might
> be easier to maintain than the separated printk_deferred() calls.
> But there might be some catches because we need to disable
> the interrupts, ...

right. and I have some doubts that people will be willing to put
alt_printk_enter/exit into those hot paths.


> Sigh, this 2nd scenario is much more likely than the 1st one.
> I guess that warnings in the scheduler/timekeeping code
> will be triggered outside printk() most of the time.

hm. may be. but the reports we received so far starts from printk()
and end up in printk() - IOW, recursion.

> It means that this approach might be much harder to sell
> after all :-(

well, it solves a number of problems that the existing implementation
cannot handle.

would have been nice to cover all of the cases, but that's a bit hard.

	-ss