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Date:   Wed, 28 Jul 2021 11:46:51 -0700
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        rcu <rcu@vger.kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        kernel-team <kernel-team@fb.com>, Ingo Molnar <mingo@kernel.org>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Josh Triplett <josh@joshtriplett.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        rostedt <rostedt@goodmis.org>,
        David Howells <dhowells@redhat.com>,
        Eric Dumazet <edumazet@google.com>,
        fweisbec <fweisbec@gmail.com>, Oleg Nesterov <oleg@redhat.com>,
        "Joel Fernandes, Google" <joel@joelfernandes.org>
Subject: Re: [PATCH v2 rcu 04/18] rcu: Weaken ->dynticks accesses and updates
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On Wed, Jul 28, 2021 at 02:12:05PM -0400, Mathieu Desnoyers wrote:
> ----- On Jul 28, 2021, at 1:58 PM, Linus Torvalds torvalds@linux-foundation.org wrote:
> 
> > On Wed, Jul 28, 2021 at 10:37 AM Paul E. McKenney <paulmck@kernel.org> wrote:
> >>
> >> +/*
> >> + * Increment the current CPU's rcu_data structure's ->dynticks field
> >> + * with ordering.  Return the new value.
> >> + */
> >> +static noinstr unsigned long rcu_dynticks_inc(int incby)
> >> +{
> >> +       return arch_atomic_add_return(incby, this_cpu_ptr(&rcu_data.dynticks));
> >> +}
> > 
> > Maybe inline?
> > 
> > But more I reacted to how we sadly don't have percpu atomics. They'd
> > be fairly easy to add on x86, but I guess it's not a huge deal.
> 
> Are the percpu atomics you have in mind different from what is found in
> Documentation/core-api/this_cpu_ops.rst ?
> 
> Namely this_cpu_add_return(pcp, val) in this case.
> 
> I must be missing something subtle because AFAIU those are already
> available. Those per-cpu atomics don't provide any memory ordering
> though, which may be why those are not used here.

Good point, but this code does indeed need the ordering.  It also must
support the occasional off-CPU access.

							Thanx, Paul