linux-kernel - Re: [PATCH RT] locking: Make spinlock_t and rwlock

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Date:   Tue, 19 Nov 2019 06:46:41 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-rt-users <linux-rt-users@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH RT] locking: Make spinlock_t and rwlock_t a RCU section
 on RT

On Tue, Nov 19, 2019 at 09:21:49AM -0500, Steven Rostedt wrote:
> On Tue, 19 Nov 2019 09:46:40 +0100
> Sebastian Andrzej Siewior <bigeasy@...utronix.de> wrote:
> 
> > On !RT a locked spinlock_t and rwlock_t disables preemption which
> > implies a RCU read section. There is code that relies on that behaviour.
> > 
> > Add an explicit RCU read section on RT while a sleeping lock (a lock
> > which would disables preemption on !RT) acquired.
> 
> I know that there was some work to merge the RCU flavors of
> rcu_read_lock and rcu_read_lock_sched, I'm assuming this depends on
> that behavior. That is, a synchronize_rcu() will wait for all CPUs to
> schedule and all grace periods to finish, which means that those using

s/grace periods/RCU readers/, but yes.

> rcu_read_lock() and those using all CPUs to schedule can be
> interchangeable. That is, on !RT, it's likely that rcu_read_lock()
> waiters will end up waiting for all CPUs to schedule, and on RT, this
> makes it where those waiting for all CPUs to schedule, will also wait
> for all rcu_read_lock()s grace periods to finish. If that's the case,
> then this change is fine. But it depends on that being the case, which
> it wasn't in older kernels, and we need to be careful about backporting
> this.

Right in one.  Backporting across the RCU flavor consolidation change
must be done with care.

							Thanx, Paul