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Subject: Re: [PATCH] printk: avoid livelock if another CPU printks
 continuously
To: Steven Rostedt <rostedt@goodmis.org>, Petr Mladek <pmladek@suse.com>
References: <1454963703-20433-1-git-send-email-dvlasenk@redhat.com>
 <20160210144416.GE12548@pathway.suse.cz>
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Cc: Denys Vlasenko <dvlasenk@redhat.com>, linux-kernel@vger.kernel.org,
        srostedt@redhat.com, Tejun Heo <tj@kernel.org>,
        Jan Kara <jack@suse.com>,
        Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Kyle McMartin <kyle@kernel.org>, KY Srinivasan <kys@microsoft.com>,
        Dave Jones <davej@codemonkey.org.uk>,
        Calvin Owens <calvinowens@fb.com>
From: Peter Hurley <peter@hurleysoftware.com>
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Date: Wed, 10 Feb 2016 08:50:46 -0800
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On 02/10/2016 08:25 AM, Steven Rostedt wrote:
> On Wed, 10 Feb 2016 17:10:16 +0100
> Petr Mladek <pmladek@suse.com> wrote:
> 
>>> Note, it's not that performance critical, and the loop only happens if
>>> someone else is adding to the console, which hopefully, should be rare.  
>>
>> I probably used too strong words. It is possible that the performance
>> impact will not be critical. But the behavior is non-deterministic.
>> I think that the approach taken by Jack is more promising.
>> I mean the offloading of the console stuff to a workqueue.
> 
> My worry about that is that it never comes out. The point about printk,
> is that it should pretty much be guaranteed to print. If the system is
> dying, and we push it off to a work queue, and that workqueue never
> runs, then we lose critical data.

I agree.

I thought a more promising approach was Pan Xinhui's patch from August [1]
which hands off console output to the incoming cpu. The reqd state machine
is described in more detail in the revised patch [2].

Unfortunately, the patch was abandoned. I think he may have misunderstood
when I also referred to Jack's patches; I meant to simply draw attention
to concurrent work for review and comparison.


[1] original https://lkml.org/lkml/2015/8/11/333
[2] revised https://lkml.org/lkml/2015/8/12/321