linux-kernel - Re: [PATCH] locking/lockdep: Increase MAX_LOCKDEP

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Date:   Tue, 26 May 2020 18:27:55 -0400
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Qian Cai <cai@....pw>, Ingo Molnar <mingo@...hat.com>,
        Will Deacon <will.deacon@....com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] locking/lockdep: Increase MAX_LOCKDEP_ENTRIES by half

On 5/26/20 5:27 PM, Peter Zijlstra wrote:
> On Tue, May 26, 2020 at 04:30:58PM -0400, Waiman Long wrote:
>> On 5/26/20 3:56 PM, Peter Zijlstra wrote:
>>> On Tue, May 26, 2020 at 02:58:50PM -0400, Qian Cai wrote:
>>>
>>>> I still don't understand why reading all sysfs files on this system
>>>> could increase that much, but here is the lockdep file after
>>>> running sysfs read to see if you could spot anything obviously,
>>>>
>>>> https://cailca.github.io/files/lockdep.txt
>>> 00000000f011a2a5 OPS:      20 FD:   45 BD:    1 .+.+: kn->active#834
>>>
>>> is that somewhere near the number of CPUs you have?
>>>
>>> Anyway, there's very long "kn->active#..." chains in there, which seems
>>> to suggest some annotation is all sorts of buggered.
>>>
>> It is actually one active lock per instance of the kerfs_node structures.
>> That means more than 800 sysfs files are accessed in some way. As we could
>> have much more than 800 sysfs files in the system, we could easily overwhelm
>> the lockdep tables if we really try to access all of them.
> A lock per instance is crazy, that's not what lockdep is made for.
> Fixing this seems like a far better idea than increasing the numbers.

Thinking about it, one lock per sysfs file does make sense as access to 
those sysfs files can invoke vastly different code path and unifying 
them into one single lock can easily lead to false positive warning.

Cheers,
Longman