linux-kernel - Re: [PATCH 3/8] srcu: Use local_lock() for per-CPU struct srcu

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Message-ID: <20200520184425.GA261674@google.com>
Date:   Wed, 20 May 2020 14:44:25 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Will Deacon <will@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Josh Triplett <josh@...htriplett.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        rcu@...r.kernel.org
Subject: Re: [PATCH 3/8] srcu: Use local_lock() for per-CPU struct srcu_data
 access

On Wed, May 20, 2020 at 08:35:29PM +0200, Peter Zijlstra wrote:
> On Wed, May 20, 2020 at 08:28:00PM +0200, Sebastian Andrzej Siewior wrote:
> > On 2020-05-20 13:42:59 [-0400], Joel Fernandes wrote:
> > > Hi Sebastian,
> > Hi Joel,
> > 
> > > For pointer stability, can we just use get_local_ptr() and put_local_ptr()
> > > instead of adding an extra lock? This keeps the pointer stable while keeping
> > > the section preemptible on -rt. And we already have a lock in rcu_data, I
> > > prefer not to add another lock if possible.
> > 
> > What is this get_local_ptr() doing? I can't find it anywhere…
> 
> I suspect it is ({ preempt_disable(); this_cpu_ptr(ptr); }), or
> something along those lines.
> 
> But yeah, I can't find it either.

I actually found it in RT 4.4 kernel, I thought this was also on newer RT
kernels as well (is that not true anymore?). But yes it was exactly what
Peter said.

In 4.4 RT there's code similar to:

#ifdef CONFIG_PREEMPT_RT_FULL

# define get_local_ptr(var) ({          \
                migrate_disable();      \
                this_cpu_ptr(var);      })

# define put_local_ptr(var) do {        \
        (void)(var);                    \
        migrate_enable();               \
} while (0)

#else

#define get_local_ptr(var)      get_cpu_ptr(var)
#define put_local_ptr(var)      put_cpu_ptr(var)

#endif


thanks,

 - Joel