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Message-ID: <571FA666.2000402@linaro.org>
Date: Tue, 26 Apr 2016 10:33:26 -0700
From: "Shi, Yang" <yang.shi@...aro.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: mingo@...hat.com, linux-kernel@...r.kernel.org,
linaro-kernel@...ts.linaro.org
Subject: Re: [PATCH] panic: lockdep: correct lock debugging state check
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
>
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