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

[Boqun Feng]

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
>