linux-kernel - Re: [PATCH tip/core/rcu 1/3] rcu: Provide polling interfaces for Tree RCU grace periods

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Message-ID: <20210316151750.GF639918@lothringen>
Date:   Tue, 16 Mar 2021 16:17:50 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     paulmck@...nel.org
Cc:     rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com, mingo@...nel.org, jiangshanlai@...il.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
        rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
        fweisbec@...il.com, oleg@...hat.com, joel@...lfernandes.org
Subject: Re: [PATCH tip/core/rcu 1/3] rcu: Provide polling interfaces for
 Tree RCU grace periods

On Wed, Mar 03, 2021 at 04:26:30PM -0800, paulmck@...nel.org wrote:
> +/**
> + * poll_state_synchronize_rcu - Conditionally wait for an RCU grace period
> + *
> + * @oldstate: return from call to get_state_synchronize_rcu() or start_poll_synchronize_rcu()
> + *
> + * If a full RCU grace period has elapsed since the earlier call from
> + * which oldstate was obtained, return @true, otherwise return @false.
> + * Otherwise, invoke synchronize_rcu() to wait for a full grace period.
> + *
> + * Yes, this function does not take counter wrap into account.
> + * But counter wrap is harmless.  If the counter wraps, we have waited for
> + * more than 2 billion grace periods (and way more on a 64-bit system!).
> + * Those needing to keep oldstate values for very long time periods
> + * (many hours even on 32-bit systems) should check them occasionally
> + * and either refresh them or set a flag indicating that the grace period
> + * has completed.
> + */
> +bool poll_state_synchronize_rcu(unsigned long oldstate)
> +{
> +	if (rcu_seq_done(&rcu_state.gp_seq, oldstate)) {
> +		smp_mb(); /* Ensure GP ends before subsequent accesses. */

Also as usual I'm a bit lost with the reason behind those memory barriers.
So this is ordering the read on rcu_state.gp_seq against something (why not an
smp_rmb() btw?). And what does it pair with?

Thanks.