linux-kernel - Re: [RFC 0/2] srcu: Remove pre-flip memory barrier

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Message-ID: <20221220225756.GB26563@lothringen>
Date:   Tue, 20 Dec 2022 23:57:56 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Joel Fernandes <joel@...lfernandes.org>,
        linux-kernel@...r.kernel.org,
        Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        "Paul E. McKenney" <paulmck@...nel.org>, rcu@...r.kernel.org,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [RFC 0/2] srcu: Remove pre-flip memory barrier

On Tue, Dec 20, 2022 at 02:01:30PM -0500, Mathieu Desnoyers wrote:
> On 2022-12-20 13:29, Joel Fernandes wrote:
> > 
> 
> > I do want to finish my memory barrier studies of SRCU over the holidays since I have been deep in the hole with that already. Back to the post flip memory barrier here since I think now even that might not be needed…
> 
> I strongly suspect the memory barrier after flip is useless for the same
> reasons I mentioned explaining why the barrier before the flip is useless.
> 
> However, we need to double-check that we have memory barriers at the
> beginning and end of synchronize_srcu, and between load of "unlock" counters
> and load of "lock" counters.
> 
> Where is the barrier at the beginning of synchronize_srcu ?

rcu_seq_snap() ?

> 
> Thanks,
> 
> Mathieu
> 
> -- 
> Mathieu Desnoyers
> EfficiOS Inc.
> https://www.efficios.com
>