linux-kernel - Re: [PATCH tip/core/rcu 7/9] rcu: Pretend ->boost

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Message-ID: <20171005095059.5hm2pumwpv75gvuk@hirez.programming.kicks-ass.net>
Date:   Thu, 5 Oct 2017 11:50:59 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     linux-kernel@...r.kernel.org, mingo@...nel.org,
        jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, rostedt@...dmis.org,
        dhowells@...hat.com, edumazet@...gle.com, fweisbec@...il.com,
        oleg@...hat.com
Subject: Re: [PATCH tip/core/rcu 7/9] rcu: Pretend ->boost_mtx acquired
 legitimately

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@...ux.vnet.ibm.com>

This is a consequence of me doing:

 f5694788ad8d ("rt_mutex: Add lockdep annotations")

Right?

> ---
>  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).