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

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Message-ID: <Y8WPWJ6TKg5ikZYr@Boquns-Mac-mini.local>
Date:   Mon, 16 Jan 2023 09:54:32 -0800
From:   Boqun Feng <boqun.feng@...il.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     paulmck@...nel.org, 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 Mon, Jan 16, 2023 at 06:36:43PM +0100, Paolo Bonzini wrote:
> 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.
> 

First, even with my change, lockdep won't report this as a deadlock
because srcu_read_lock() is annotated as a recursive (unfair) read lock
(the "read" parameter for lock_acquire() is 2) and in this case lockdep
knows that it won't cause deadlock.

For SRCU, the annotation mapping that is 1) srcu_read_lock() is marked
as recursive read lock and 2) synchronize_srcu() is marked as write lock
sync, recursive read locks themselves cannot cause deadlocks and lockdep
is aware of it.

Will add a selftest for this later.

> 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

To be more precise, to have a deadlock, the read lock on CPU 0 has to be
a *fair* read lock (i.e. non-recursive reader, the "read" parameter for
lock_acquire is 1)

> correctly suppress this?
> 

I'm pretty sure that lockdep handles this ;-)

Regards,
Boqun

> Paolo
>