linux-kernel - Re: [PATCH v2] lockdep: upper limit LOCKDEP_CHAINS

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Message-ID: <5178.1722556282@jrotkm2>
Date: Fri, 02 Aug 2024 08:51:22 +0900
From: "J. R. Okajima" <hooanon05g@...il.com>
To: Carlos Llamas <cmllamas@...gle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
    linux-kernel@...r.kernel.org, kernel-team@...roid.com,
    Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
    Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v2] lockdep: upper limit LOCKDEP_CHAINS_BITS

Carlos Llamas:
> I simply dug up this patch from J.R. which avoids the BUILD_BUG_ON(),
> but perhaps someone should limit the rest of the configs? In practice,
> nobody should be using these 30 bit shifts.

I posted the patch in 2021. It was against the commit in v5.13-rc1,
        5dc33592e9553 2021-04-05 lockdep: Allow tuning tracing capacity constants.
It is a little surprise for me that you could pick up such old post.

As I wrote in the old post
	https://lore.kernel.org/all/30795.1620913191@jrobl/
	I don't know what this 'multiply by 5' means and why
	ARRAY_SIZE(chain_hlocks) is limited to (1UL << 24)
And the post is just to silence BUILD_BUG_ON().
There are a few more "range 10 30" in lib/Kconfig.debug.


> > btw, the help text "Bitsize for MAX_LOCKDEP_CHAINS" is odd.  What's a
> > bitsize?  Maybe "bit shift count for..." or such.
>
> Indeed that is odd. I'm also not sure what to make of the "*5" magic
> number. I suppose it could be the typical lock depth? I could try to
> clarify these points, if no one with more insight wants to do it.

Also the original text comes from the commit in v5.13-rc1.


J. R. Okajima