linux-kernel - Re: [PATCH v5 0/2] printk: Console owner and waiter logic cleanup

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Date:   Thu, 11 Jan 2018 10:34:35 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>, Tejun Heo <tj@...nel.org>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        akpm@...ux-foundation.org, linux-mm@...ck.org,
        Cong Wang <xiyou.wangcong@...il.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Mel Gorman <mgorman@...e.de>, Michal Hocko <mhocko@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jan Kara <jack@...e.cz>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        rostedt@...e.goodmis.org, Byungchul Park <byungchul.park@....com>,
        Pavel Machek <pavel@....cz>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 0/2] printk: Console owner and waiter logic cleanup

On Thu 2018-01-11 13:58:17, Sergey Senozhatsky wrote:
> On (01/10/18 13:05), Steven Rostedt wrote:
> > The solution is simple, everyone at KS agreed with it, there should be
> > no controversy here.
> 
> frankly speaking, that's not what I recall ;)

To be honest, I do not longer remember the details. I think that
nobody was really against that solution. Of course, there were
doubts and other proposals.

I think that I was actually the most sceptical guy there. I would
split my old doubts into three areas:

      + new possible deadlocks
            -> I was wrong

      + did not fully prevent softlockups
            -> no real life example in hands

      + looked tricky and complex
	    -> like many other new things

You see that I have changed my mind and decided to give this solution
a chance.

 
> [..]
> > My printk solution is solid, with no risk of regressions of current
> > printk usages.
> 
> except that handing off a console_sem to atomic task when there
> is   O(logbuf) > watchdog_thresh   is a regression, basically...
> it is what it is.

How this could be a regression? Is not the victim that handles
other printk's random? What protected the atomic task to
handle the other printks before this patch?

Or do you have a system that started to suffer from softlockups
with this patchset and did not do this before?
 
> 
> > If anything, I'll pull theses patches myself, and push them to Linus
> > directly
> 
> lovely.

Do you know about any system where this patch made the softlockup
deterministically or statistically more likely, please?

Best Regards,
Petr