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

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Date:   Tue, 20 Dec 2022 14:01:30 -0500
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     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 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 ?

Thanks,

Mathieu

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