Re: RCU use of swait

From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To: Joel Fernandes <joel@joelfernandes.org>
Cc: "Paul E. McKenney" <paulmck@kernel.org>,
	Josh Triplett <josh@joshtriplett.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	Lai Jiangshan <jiangshanlai@gmail.com>,
	Thomas Glexiner <tglx@linutronix.de>,
	Scott Wood <swood@redhat.com>,
	Clark Williams <williams@redhat.com>,
	Juri Lelli <juri.lelli@redhat.com>,
	linux-rt-users@vger.kernel.org, rcu <rcu@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Ingo Molnar <mingo@kernel.org>
Subject: Re: RCU use of swait
Date: Thu, 19 Mar 2020 10:25:40 +0100	[thread overview]
Message-ID: <20200319092540.gpgzx2qao3r4okql@linutronix.de> (raw)
In-Reply-To: <CAEXW_YSiKrT50mTR1a4tB5x_jQzZKnuAo6JY-Vc5w7r=XLv+OA@mail.gmail.com>

On 2020-03-18 20:47:18 [-0400], Joel Fernandes wrote:
> In RCU, there are some truly-atomic code sections containing an
> swake_upXX() call, which would be considered atomic also on
> PREEMPT_RT, one example is:
> 
> rcu_core() contains an local_irq_{save,restore}() section.
> 
>         /* No grace period and unregistered callbacks? */
>         if (!rcu_gp_in_progress() &&
>             rcu_segcblist_is_enabled(&rdp->cblist) && !offloaded) {
>                 local_irq_save(flags);
>                 if (!rcu_segcblist_restempty(&rdp->cblist, RCU_NEXT_READY_TAIL))
>                         rcu_accelerate_cbs_unlocked(rnp, rdp);
>                 local_irq_restore(flags);
>         }
> 
> And rcu_accelerate_cbs_unlocked(rnp, rdp) calls rcu_gp_kthread_wake()
> which does an swake_up_one(), so I think we will have to leave it as
> swake_up() the way it is.

There is also this:
irq_exit()
 rcu_irq_exit()
  rcu_nmi_exit_common(true);
   rcu_prepare_for_idle()
    if (rcu_segcblist_pend_cbs(&rdp->cblist))
      rcu_gp_kthread_wake()
       swake_up_one(&rcu_state.gp_wq);

So I *think* this is another one.

> thanks,
> 
>  - Joel

Sebastian