linux-kernel - Re: [PATCH RT v2 2/3] sched: migrate_enable: Use sleeping

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Date:   Wed, 21 Aug 2019 16:35:55 -0700
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     Scott Wood <swood@...hat.com>
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        linux-rt-users@...r.kernel.org, linux-kernel@...r.kernel.org,
        Joel Fernandes <joel@...lfernandes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Clark Williams <williams@...hat.com>
Subject: Re: [PATCH RT v2 2/3] sched: migrate_enable: Use sleeping_lock to
 indicate involuntary sleep

On Wed, Aug 21, 2019 at 06:19:05PM -0500, Scott Wood wrote:
> Without this, rcu_note_context_switch() will complain if an RCU read
> lock is held when migrate_enable() calls stop_one_cpu().
> 
> Signed-off-by: Scott Wood <swood@...hat.com>

I have to ask...  Both sleeping_lock_inc() and sleeping_lock_dec() are
no-ops if not CONFIG_PREEMPT_RT_BASE?

							Thanx, Paul

> ---
> v2: Added comment.
> 
> If my migrate disable changes aren't taken, then pin_current_cpu()
> will also need to use sleeping_lock_inc() because calling
> __read_rt_lock() bypasses the usual place it's done.
> 
>  include/linux/sched.h    | 4 ++--
>  kernel/rcu/tree_plugin.h | 2 +-
>  kernel/sched/core.c      | 8 ++++++++
>  3 files changed, 11 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 7e892e727f12..1ebc97f28009 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -673,7 +673,7 @@ struct task_struct {
>  	int				migrate_disable_atomic;
>  # endif
>  #endif
> -#ifdef CONFIG_PREEMPT_RT_FULL
> +#ifdef CONFIG_PREEMPT_RT_BASE
>  	int				sleeping_lock;
>  #endif
>  
> @@ -1881,7 +1881,7 @@ static __always_inline bool need_resched(void)
>  	return unlikely(tif_need_resched());
>  }
>  
> -#ifdef CONFIG_PREEMPT_RT_FULL
> +#ifdef CONFIG_PREEMPT_RT_BASE
>  static inline void sleeping_lock_inc(void)
>  {
>  	current->sleeping_lock++;
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index 23a54e4b649c..7a3aa085ce2c 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -292,7 +292,7 @@ void rcu_note_context_switch(bool preempt)
>  	barrier(); /* Avoid RCU read-side critical sections leaking down. */
>  	trace_rcu_utilization(TPS("Start context switch"));
>  	lockdep_assert_irqs_disabled();
> -#if defined(CONFIG_PREEMPT_RT_FULL)
> +#if defined(CONFIG_PREEMPT_RT_BASE)
>  	sleeping_l = t->sleeping_lock;
>  #endif
>  	WARN_ON_ONCE(!preempt && t->rcu_read_lock_nesting > 0 && !sleeping_l);
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index e1bdd7f9be05..0758ee85634e 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -7405,7 +7405,15 @@ void migrate_enable(void)
>  			unpin_current_cpu();
>  			preempt_lazy_enable();
>  			preempt_enable();
> +
> +			/*
> +			 * sleeping_lock_inc suppresses a debug check for
> +			 * sleeping inside an RCU read side critical section
> +			 */
> +			sleeping_lock_inc();
>  			stop_one_cpu(task_cpu(p), migration_cpu_stop, &arg);
> +			sleeping_lock_dec();
> +
>  			return;
>  		}
>  	}
> -- 
> 1.8.3.1
>