linux-kernel - Re: [PATCH RT v3 4/5] rcu: Disable use_softirq on PREEMPT

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Message-ID: <aae7d8cfa23485127e7c404d5fe8bdf8a70627c8.camel@redhat.com>
Date:   Tue, 17 Sep 2019 09:08:45 -0500
From:   Scott Wood <swood@...hat.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
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>,
        Steven Rostedt <rostedt@...dmis.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Clark Williams <williams@...hat.com>
Subject: Re: [PATCH RT v3 4/5] rcu: Disable use_softirq on PREEMPT_RT

On Wed, 2019-09-11 at 17:57 +0100, Scott Wood wrote:
>  kernel/rcu/tree.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index fc8b00c61b32..ee0a5ec2c30f 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -98,9 +98,14 @@ struct rcu_state rcu_state = {
>  /* Dump rcu_node combining tree at boot to verify correct setup. */
>  static bool dump_tree;
>  module_param(dump_tree, bool, 0444);
> -/* By default, use RCU_SOFTIRQ instead of rcuc kthreads. */
> -static bool use_softirq = 1;
> +/*
> + * By default, use RCU_SOFTIRQ instead of rcuc kthreads.
> + * But, avoid RCU_SOFTIRQ on PREEMPT_RT due to pi/rq deadlocks.
> + */
> +static bool use_softirq = !IS_ENABLED(CONFIG_PREEMPT_RT_FULL);
> +#ifdef CONFIG_PREEMPT_RT_FULL
>  module_param(use_softirq, bool, 0444);
> +#endif

Ugh, that should be s/ifdef/ifndef/

-Scott