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

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Message-ID: <20191119092149.06fd8f87@gandalf.local.home>
Date:   Tue, 19 Nov 2019 09:21:49 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        linux-rt-users <linux-rt-users@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Paul E. McKenney" <paulmck@...nel.org>
Subject: Re: [PATCH RT] locking: Make spinlock_t and rwlock_t a RCU section
 on RT

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
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.

-- Steve

> 
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
>