linux-kernel - Re: [PATCH v2 1/4] locking/mutex: implement mutex_trylock

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Message-ID: <20250410080454.GW9833@noisy.programming.kicks-ass.net>
Date: Thu, 10 Apr 2025 10:04:54 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Maxim Levitsky <mlevitsk@...hat.com>
Cc: kvm@...r.kernel.org, Alexander Potapenko <glider@...gle.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Suzuki K Poulose <suzuki.poulose@....com>,
	kvm-riscv@...ts.infradead.org,
	Oliver Upton <oliver.upton@...ux.dev>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Jing Zhang <jingzhangos@...gle.com>,
	Waiman Long <longman@...hat.com>, x86@...nel.org,
	Kunkun Jiang <jiangkunkun@...wei.com>,
	Boqun Feng <boqun.feng@...il.com>, Anup Patel <anup@...infault.org>,
	Albert Ou <aou@...s.berkeley.edu>, kvmarm@...ts.linux.dev,
	linux-kernel@...r.kernel.org, Zenghui Yu <yuzenghui@...wei.com>,
	Borislav Petkov <bp@...en8.de>, Alexandre Ghiti <alex@...ti.fr>,
	Keisuke Nishimura <keisuke.nishimura@...ia.fr>,
	Sebastian Ott <sebott@...hat.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Atish Patra <atishp@...shpatra.org>,
	Paul Walmsley <paul.walmsley@...ive.com>,
	Randy Dunlap <rdunlap@...radead.org>, Will Deacon <will@...nel.org>,
	Palmer Dabbelt <palmer@...belt.com>,
	linux-riscv@...ts.infradead.org, Marc Zyngier <maz@...nel.org>,
	linux-arm-kernel@...ts.infradead.org,
	Joey Gouly <joey.gouly@....com>, Ingo Molnar <mingo@...hat.com>,
	Andre Przywara <andre.przywara@....com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Sean Christopherson <seanjc@...gle.com>,
	Catalin Marinas <catalin.marinas@....com>,
	Bjorn Helgaas <bhelgaas@...gle.com>
Subject: Re: [PATCH v2 1/4] locking/mutex: implement mutex_trylock_nested

On Tue, Apr 08, 2025 at 09:41:33PM -0400, Maxim Levitsky wrote:
> Allow to specify the lockdep subclass in mutex_trylock
> instead of hardcoding it to 0.

We disable a whole bunch of checks for trylock, simply because they do
not wait, therefore they cannot deadlock.

But I can't remember if they disable all the cases required to make
subclasses completely redundant -- memory suggests they do, but I've not
verified.

Please expand this Changelog to include definite proof that subclasses
make sense with trylock.