linux-kernel - Re: [RFC PATCH 00/12] locking/lockdep: Add a new class of terminal locks

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Message-ID: <dfa0a2fa-0094-3ae0-4f27-2930233132a3@redhat.com>
Date:   Sat, 10 Nov 2018 18:35:57 -0500
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Will Deacon <will.deacon@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org, kasan-dev@...glegroups.com,
        linux-mm@...ck.org, Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Tejun Heo <tj@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [RFC PATCH 00/12] locking/lockdep: Add a new class of terminal
 locks

On 11/10/2018 09:10 AM, Peter Zijlstra wrote:
> On Fri, Nov 09, 2018 at 09:04:12AM +0100, Ingo Molnar wrote:
>> BTW., if you are interested in more radical approaches to optimize 
>> lockdep, we could also add a static checker via objtool driven call graph 
>> analysis, and mark those locks terminal that we can prove are terminal.
>>
>> This would require the unified call graph of the kernel image and of all 
>> modules to be examined in a final pass, but that's within the principal 
>> scope of objtool. (This 'final pass' could also be done during bootup, at 
>> least in initial versions.)
> Something like this is needed for objtool LTO support as well. I just
> dread the build time 'regressions' this will introduce :/
>
> The final link pass is already by far the most expensive part (as
> measured in wall-time) of building a kernel, adding more work there
> would really suck :/

I think the idea is to make objtool have the capability to do that. It
doesn't mean we need to turn it on by default in every build.

Cheers,
Longman