Subject: Re: [PATCH] panic: lockdep: correct lock debugging state check
To: Peter Zijlstra <peterz@infradead.org>
References: <1461641797-9368-1-git-send-email-yang.shi@linaro.org>
 <20160426123955.GT3448@twins.programming.kicks-ass.net>
Cc: mingo@redhat.com, linux-kernel@vger.kernel.org,
        linaro-kernel@lists.linaro.org
From: "Shi, Yang" <yang.shi@linaro.org>
Message-ID: <571FA666.2000402@linaro.org>
Date: Tue, 26 Apr 2016 10:33:26 -0700
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On 4/26/2016 5:39 AM, Peter Zijlstra wrote:
> On Mon, Apr 25, 2016 at 08:36:37PM -0700, Yang Shi wrote:
>> When kernel oops happens, lock debugging is turned off by debug_locks_off()
>> in oops_enter() via calling __debug_locks_off() which set debug_locks to 0
>> via xchg(). But, calling to __debug_locks_off() to check lock debugging state
>> in add_taint() called by oops_end() will always return false since xchg()
>> returns the old value of debug_locks which is cleared in oops_enter() already.
>>
>> This prevents add_taint() from printing out lock debugging disable information
>> although LOCKDEP_NOW_UNRELIABLE is passed to it.
>>
>> Check lock debugging state via !debug_locks to fix this. Although
>> !__debug_locks_off() could do the same thing, it may look confusing.
>>
> What are you smoking? This is the second completely insane patch you
> send this week.
>
> This breaks add_taint() and gains us nothing except trivialities. Who

I apologize in advance, if I misunderstand the code and please ignore 
all the bullshit below.

In my understanding, add_taint() should call that pr_warn if 
LOCKDEP_NOW_UNRELIABLE is passed and lock debugging is disabled. This is 
what the code tells me.

LOCKDEP_NOW_UNRELIABLE is passed via lock_ok parameter, lock debugging 
is turned off by debug_locks_off() already, so it should print out 
something, but it doesn't since __debug_locks_off() always returns 0.

So, it looks the if statement logic is broken.

There are alternatives to fix it, I may pick up the not ideal one.

> bloody cares about that print if you've just had an OOPS.

I do agree not too many people care about that print and such 
information is too trivial to draw attention from people. However, it 
doesn't mean oops print is a perfect place to hide something wrong. I 
just happened to find this by checking the oops information to try to 
get some clue for another issue. Then I thought it is just a quick fix, 
why not I should do that to make kernel better.

Thanks,
Yang

>