linux-kernel - Re: [PATCH rcu 4/7] locking/lockdep: Improve the deadlock scenario print for sync and read lock

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Date:   Mon, 20 Mar 2023 10:50:03 -0700
From:   Boqun Feng <boqun.feng@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     rcu@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Will Deacon <will@...nel.org>,
        Waiman Long <longman@...hat.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Josh Triplett <josh@...htriplett.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Frederic Weisbecker <frederic@...nel.org>,
        Neeraj Upadhyay <quic_neeraju@...cinc.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Shuah Khan <shuah@...nel.org>,
        David Woodhouse <dwmw2@...radead.org>,
        Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        seanjc@...gle.com, linux-kernel@...r.kernel.org,
        linux-kselftest@...r.kernel.org
Subject: Re: [PATCH rcu 4/7] locking/lockdep: Improve the deadlock scenario
 print for sync and read lock

On Mon, Mar 20, 2023 at 01:13:05PM +0100, Peter Zijlstra wrote:
> On Thu, Mar 16, 2023 at 08:13:36PM -0700, Boqun Feng wrote:
> > Lock scenario print is always a weak spot of lockdep splats. Improvement
> > can be made if we rework the dependency search and the error printing.
> > 
> > However without touching the graph search, we can improve a little for
> > the circular deadlock case, since we have the to-be-added lock
> > dependency, and know whether these two locks are read/write/sync.
> > 
> > In order to know whether a held_lock is sync or not, a bit was
> > "stolen" from ->references, which reduce our limit for the same lock
> > class nesting from 2^12 to 2^11, and it should still be good enough.
> > 
> > Besides, since we now have bit in held_lock for sync, we don't need the
> > "hardirqoffs being 1" trick, and also we can avoid the __lock_release()
> > if we jump out of __lock_acquire() before the held_lock stored.
> > 
> > With these changes, a deadlock case evolved with read lock and sync gets
> > a better print-out from:
> > 
> > 	[...]  Possible unsafe locking scenario:
> > 	[...]
> > 	[...]        CPU0                    CPU1
> > 	[...]        ----                    ----
> > 	[...]   lock(srcuA);
> > 	[...]                                lock(srcuB);
> > 	[...]                                lock(srcuA);
> > 	[...]   lock(srcuB);
> > 
> > to
> > 
> > 	[...]  Possible unsafe locking scenario:
> > 	[...]
> > 	[...]        CPU0                    CPU1
> > 	[...]        ----                    ----
> > 	[...]   rlock(srcuA);
> > 	[...]                                lock(srcuB);
> > 	[...]                                lock(srcuA);
> > 	[...]   sync(srcuB);
> > 
> > Signed-off-by: Boqun Feng <boqun.feng@...il.com>
> > Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> > Signed-off-by: Boqun Feng <boqun.feng@...il.com>
> > ---
> >  include/linux/lockdep.h  |  3 ++-
> >  kernel/locking/lockdep.c | 48 ++++++++++++++++++++++++++--------------
> >  2 files changed, 34 insertions(+), 17 deletions(-)
> > 
> > diff --git a/include/linux/lockdep.h b/include/linux/lockdep.h
> > index 14d9dbedc6c1..b32256e9e944 100644
> > --- a/include/linux/lockdep.h
> > +++ b/include/linux/lockdep.h
> > @@ -134,7 +134,8 @@ struct held_lock {
> >  	unsigned int read:2;        /* see lock_acquire() comment */
> >  	unsigned int check:1;       /* see lock_acquire() comment */
> >  	unsigned int hardirqs_off:1;
> > -	unsigned int references:12;					/* 32 bits */
> > +	unsigned int sync:1;
> > +	unsigned int references:11;					/* 32 bits */
> >  	unsigned int pin_count;
> >  };
> >  
> 
> Yeah, I suppose we can do that -- another option is to steal some bits
> from pin_count, but whatever (references used to be 11 a long while ago,
> no problem going back to that).

Thanks!

> 
> Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>

Applied locally.

Regards,
Boqun