Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Sat, 12 Mar 2022 23:54:09 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Frederic Weisbecker <frederic@kernel.org>
Cc:     LKML <linux-kernel@vger.kernel.org>, Phil Auld <pauld@redhat.com>,
        Alex Belits <abelits@marvell.com>,
        Nicolas Saenz Julienne <nsaenz@kernel.org>,
        Xiongfeng Wang <wangxiongfeng2@huawei.com>,
        Neeraj Upadhyay <quic_neeraju@quicinc.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Yu Liao <liaoyu15@huawei.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        "Paul E . McKenney" <paulmck@kernel.org>,
        Marcelo Tosatti <mtosatti@redhat.com>,
        Paul Gortmaker <paul.gortmaker@windriver.com>,
        Uladzislau Rezki <uladzislau.rezki@sony.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Will Deacon <will@kernel.org>
Subject: Re: [PATCH 16/19] context_tracking: Convert state to atomic_t
Message-ID: <20220312225409.GH28057@worktop.programming.kicks-ass.net>
References: <20220302154810.42308-1-frederic@kernel.org>
 <20220302154810.42308-17-frederic@kernel.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20220302154810.42308-17-frederic@kernel.org>
Precedence: bulk

On Wed, Mar 02, 2022 at 04:48:07PM +0100, Frederic Weisbecker wrote:
> +static __always_inline int __ct_state(void)
> +{
> +	return atomic_read(this_cpu_ptr(&context_tracking.state));
> +}

One arguably horrible thing to do would be to write it like:

	return __this_cpu_read(context_tracking.state.counter);

IIRC that will actually DTRT since atomic_read() is basically defined to
be READ_ONCE() and this_cpu_read() implies the same.

Only PowerPC and s390 implement arch_atomic_read() in asm, but I don't
think they have a particularly good reason to. The only other weird case
is Alpha, where READ_ONCE() implies smp_mb() because Alpha. I'm not sure
we care about that case, hmm?

The same can be done for ct_dynticks(), which is basically the same
function with a different mask.

As mentioned elsewhere, ct_state() appears unused at the end of the
ride.