Re: [PATCH tip/core/rcu 7/9] rcu: Pretend ->boost_mtx acquired legitimately

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Thu, Oct 05, 2017 at 11:50:59AM +0200, Peter Zijlstra wrote:
> On Wed, Oct 04, 2017 at 02:29:33PM -0700, Paul E. McKenney wrote:
> > RCU priority boosting uses rt_mutex_init_proxy_locked() to initialize an
> > rt_mutex structure in locked state held by some other task.  When that
> > other task releases it, lockdep complains (quite accurately, but a bit
> > uselessly) that the other task never acquired it.  This complaint can
> > suppress other, more helpful, lockdep complaints, and in any case it is
> > a false positive.
> > 
> > This commit therefore uses the mutex_acquire() macro to make it look
> > like that other process legitimately acquired the lock, thus suppressing
> > this lockdep false-positive complaint.
> > 
> > Of course, if lockdep ever learns about rt_mutex_init_proxy_locked(),
> > this commit will need to be reverted.
> > 
> > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> 
> This is a consequence of me doing:
> 
>  f5694788ad8d ("rt_mutex: Add lockdep annotations")
> 
> Right?

The timing matches, so I do believe this is the case.

> > ---
> >  kernel/rcu/tree_plugin.h | 5 ++++-
> >  1 file changed, 4 insertions(+), 1 deletion(-)
> > 
> > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> > index fed95fa941e6..60bfb16c9a1a 100644
> > --- a/kernel/rcu/tree_plugin.h
> > +++ b/kernel/rcu/tree_plugin.h
> > @@ -529,8 +529,11 @@ void rcu_read_unlock_special(struct task_struct *t)
> >  		}
> >  
> >  		/* Unboost if we were boosted. */
> > -		if (IS_ENABLED(CONFIG_RCU_BOOST) && drop_boost_mutex)
> > +		if (IS_ENABLED(CONFIG_RCU_BOOST) && drop_boost_mutex) {
> > +			/* For lockdep, pretend we acquired lock honestly. */
> > +			mutex_acquire(&rnp->boost_mtx.dep_map, 0, 0, _RET_IP_);
> >  			rt_mutex_unlock(&rnp->boost_mtx);
> > +		}
> 
> So I'm thinking the problem is that you're mixing rt_mutex and PI-futex
> primitives here. As per commit:
> 
>   5293c2efda37 ("futex,rt_mutex: Provide futex specific rt_mutex API")
> 
> these are two separate APIs, that should, ideally, not be mixed.
> 
> The 'right' counterpart to rt_mutex_init_proxy_locked() is
> rt_mutex_futex_unlock() (which very much does not include lockdep bits).

OK, will give this a try.  It does at least seem to build, so I guess
that is a good start.  ;-)

							Thanx, Paul