linux-kernel - Re: [PATCH 2/3] rcu: Equip sleepable RCU with lockdep dependency graph checks

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Message-ID: <456f6c15-3043-6da2-349d-c0c3880c1a55@redhat.com>
Date:   Mon, 16 Jan 2023 18:36:43 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     paulmck@...nel.org, Boqun Feng <boqun.feng@...il.com>
Cc:     linux-kernel@...r.kernel.org, rcu@...r.kernel.org,
        kvm@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Waiman Long <longman@...hat.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Josh Triplett <josh@...htriplett.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        David Woodhouse <dwmw2@...radead.org>, seanjc@...gle.com,
        Joel Fernandes <joel@...lfernandes.org>,
        Matthew Wilcox <willy@...radead.org>,
        Michal Luczaj <mhal@...x.co>
Subject: Re: [PATCH 2/3] rcu: Equip sleepable RCU with lockdep dependency
 graph checks

On 1/13/23 20:11, Paul E. McKenney wrote:
> On Fri, Jan 13, 2023 at 10:05:22AM -0800, Boqun Feng wrote:
>> On Fri, Jan 13, 2023 at 03:29:49AM -0800, Paul E. McKenney wrote:
>> I prefer that the first two patches go through your tree, because it
>> reduces the synchronization among locking, rcu and KVM trees to the
>> synchronization betwen rcu and KVM trees.
> 
> Very well, I have queued and pushed these with the usual wordsmithing,
> thank you!

I'm worried about this case:

	CPU 0				CPU 1
	--------------------		------------------
	lock A				srcu lock B
	srcu lock B			lock A
	srcu unlock B			unlock A
	unlock A			srcu unlock B

While a bit unclean, there is nothing that downright forbids this; as 
long as synchronize_srcu does not happen inside lock A, no deadlock can 
occur.

However, if srcu is replaced with an rwlock then lockdep should and does 
report a deadlock.  Boqun, do you get a false positive or do your 
patches correctly suppress this?

Paolo