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From:   Joel Fernandes <joel@joelfernandes.org>
Date:   Wed, 18 Mar 2020 20:47:18 -0400
Message-ID: <CAEXW_YSiKrT50mTR1a4tB5x_jQzZKnuAo6JY-Vc5w7r=XLv+OA@mail.gmail.com>
Subject: Re: RCU use of swait
To:     Sebastian Andrzej Siewior <bigeasy@linutronix.de>
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>
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On Thu, Mar 5, 2020 at 3:11 AM Sebastian Andrzej Siewior
<bigeasy@linutronix.de> wrote:
>
> On 2020-03-04 16:35:26 [-0800], Paul E. McKenney wrote:
> > Hello!
> Hi Paul,
>
> > Or is there some other reason why {S,}RCU needs to use swait that I
> > am forgetting?
>
> swait can be used in hardirq-context (on RT) that is in a section within
> local_irq_save() / raw_spin_lock() and so.
> wait on the hand uses spinlock_t which can not be used in this context.
> So if you have no users which fall into that category then you could
> move back to wait.h.

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.

thanks,

 - Joel