linux-kernel - Re: [patch V2 18/29] lockdep: Move stack trace logic into check_prev

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Message-ID: <alpine.DEB.2.21.1904242148480.1762@nanos.tec.linutronix.de>
Date:   Wed, 24 Apr 2019 21:51:05 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>
cc:     LKML <linux-kernel@...r.kernel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>, x86@...nel.org,
        Andy Lutomirski <luto@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Alexander Potapenko <glider@...gle.com>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Pekka Enberg <penberg@...nel.org>, linux-mm@...ck.org,
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        Christoph Lameter <cl@...ux.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
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        linux-arch@...r.kernel.org
Subject: Re: [patch V2 18/29] lockdep: Move stack trace logic into
 check_prev_add()

On Wed, 24 Apr 2019, Peter Zijlstra wrote:
> On Thu, Apr 18, 2019 at 10:41:37AM +0200, Thomas Gleixner wrote:
> > There is only one caller of check_prev_add() which hands in a zeroed struct
> > stack trace and a function pointer to save_stack(). Inside check_prev_add()
> > the stack_trace struct is checked for being empty, which is always
> > true. Based on that one code path stores a stack trace which is unused. The
> > comment there does not make sense either. It's all leftovers from
> > historical lockdep code (cross release).
> 
> I was more or less expecting a revert of:
> 
> ce07a9415f26 ("locking/lockdep: Make check_prev_add() able to handle external stack_trace")
> 
> And then I read the comment that went with the "static struct
> stack_trace trace" that got removed (in the above commit) and realized
> that your patch will consume more stack entries.
> 
> The problem is when the held lock stack in check_prevs_add() has multple
> trylock entries on top, in that case we call check_prev_add() multiple
> times, and this patch will then save the exact same stack-trace multiple
> times, consuming static resources.
> 
> Possibly we should copy what stackdepot does (but we cannot use it
> directly because stackdepot uses locks; but possible we can share bits),
> but that is a patch for another day I think.
> 
> So while convoluted, perhaps we should retain this code for now.

Uurg, what a mess.