linux-kernel - Re: [PATCH] percpu-refcount: Use normal instead of RCU-sched"

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Date:   Wed, 2 Oct 2019 17:17:19 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     linux-kernel@...r.kernel.org, Dennis Zhou <dennis@...nel.org>,
        Tejun Heo <tj@...nel.org>, Christoph Lameter <cl@...ux.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] percpu-refcount: Use normal instead of RCU-sched"

On 2019-10-02 08:08:52 [-0700], Paul E. McKenney wrote:
> On Wed, Oct 02, 2019 at 01:22:53PM +0200, Sebastian Andrzej Siewior wrote:
> > This is a revert of commit
> >    a4244454df129 ("percpu-refcount: use RCU-sched insted of normal RCU")
> > 
> > which claims the only reason for using RCU-sched is
> >    "rcu_read_[un]lock() … are slightly more expensive than preempt_disable/enable()"
> > 
> > and
> >     "As the RCU critical sections are extremely short, using sched-RCU
> >     shouldn't have any latency implications."
> > 
> > The problem with using RCU-sched here is that it disables preemption and
> > the callback must not acquire any sleeping locks like spinlock_t on
> > PREEMPT_RT which is the case with some of the users.
> 
> Looks good in general, but changing to RCU-preempt does not change the
> fact that the callbacks execute with bh disabled.  There is a newish
> queue_rcu_work() that invokes a workqueue handler after a grace period.
> 
> Or am I missing your point here?

That is fine, no the RCU callback. The problem is that
percpu_ref_put_many() as of now does:

	rcu_read_lock_sched(): /* aka preempt_disable(); */
	if (__ref_is_percpu(ref, &percpu_count))
		this_cpu_sub(*percpu_count, nr);
	else if (unlikely(atomic_long_sub_and_test(nr, &ref->count)))
		ref->release(ref);

and then the callback invoked via ref->release() acquires a spinlock_t
with disabled preemption.
 
> 							Thanx, Paul

Sebastian