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Date:   Mon, 16 Jan 2023 18:36:43 +0100
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Subject: Re: [PATCH 2/3] rcu: Equip sleepable RCU with lockdep dependency
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To:     paulmck@kernel.org, Boqun Feng <boqun.feng@gmail.com>
Cc:     linux-kernel@vger.kernel.org, rcu@vger.kernel.org,
        kvm@vger.kernel.org, Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>, Will Deacon <will@kernel.org>,
        Waiman Long <longman@redhat.com>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        Josh Triplett <josh@joshtriplett.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        David Woodhouse <dwmw2@infradead.org>, seanjc@google.com,
        Joel Fernandes <joel@joelfernandes.org>,
        Matthew Wilcox <willy@infradead.org>,
        Michal Luczaj <mhal@rbox.co>
References: <20230113065955.815667-1-boqun.feng@gmail.com>
 <20230113065955.815667-3-boqun.feng@gmail.com>
 <20230113112949.GX4028633@paulmck-ThinkPad-P17-Gen-1>
 <Y8GdYgSBtyKwf/qj@boqun-archlinux>
 <20230113191120.GB4028633@paulmck-ThinkPad-P17-Gen-1>
From:   Paolo Bonzini <pbonzini@redhat.com>
In-Reply-To: <20230113191120.GB4028633@paulmck-ThinkPad-P17-Gen-1>
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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