linux-kernel - Re: [PATCH] printk: avoid livelock if another CPU printks continuously

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Message-ID: <56BA16B5.60805@redhat.com>
Date:	Tue, 09 Feb 2016 17:41:25 +0100
From:	Denys Vlasenko <dvlasenk@...hat.com>
To:	Steven Rostedt <rostedt@...dmis.org>
CC:	linux-kernel@...r.kernel.org, srostedt@...hat.com,
	Tejun Heo <tj@...nel.org>,
	Peter Hurley <peter@...leysoftware.com>
Subject: Re: [PATCH] printk: avoid livelock if another CPU printks continuously

On 02/09/2016 04:50 PM, Steven Rostedt wrote:
> On Tue, 09 Feb 2016 16:24:29 +0100
>>>>>> +				/* Good, other CPU entered "for(;;)" loop */
>>>>>> +				goto out;
>>>>>> +			}
>>>>>> +		}
>>>>>> +		/* No one seems to be willing to take it... */
>>>>>> +		if (console_trylock())
>>>>>> +			goto again; /* we took it */    
>>>>>
>>>>> Perhaps add a few loops to the taking of the console sem.    
>>>>
>>>> Why?
>>>>
>>>> If we fail to take the lock, another CPU took it.
>>>> There is no need to try harder, as soon as we know that any
>>>> other CPU took that lock, we can safely exit this function.  
>>>
>>> Because if this CPU is the one spamming the other CPU, it will widen
>>> the window to be the one that takes the lock.  
>>
>> If we reached this code, we aren't the spamming CPU. We are the CPU
>> which is being spammed (we are in the loop which services the backlog).
> 
> No, I mentioned the taking of console sem. The spamming task will be
> trying that a bit, failing and then letting this CPU continue doing its
> bidding.

That's exactly what we *don't* want to happen.
We want that other CPU to get the lock.

How do you plan to achieve that, if not by giving it a grace period
when it can grab a lock?