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

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Message-ID: <20190823162024.47t7br6ecfclzgkw@linutronix.de>
Date:   Fri, 23 Aug 2019 18:20:24 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Scott Wood <swood@...hat.com>
Cc:     linux-rt-users@...r.kernel.org, linux-kernel@...r.kernel.org,
        "Paul E . McKenney" <paulmck@...ux.ibm.com>,
        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 2019-08-21 18:19:05 [-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>
> ---
> 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(-)
> 
> --- 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();

this looks like an ugly hack. This sleeping_lock_inc() is used where we
actually hold a sleeping lock and schedule() which is okay. But this
would mean we hold a RCU lock and schedule() anyway. Is that okay?

> +
>  			return;
>  		}
>  	}

Sebastian