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Date:   Fri, 20 Jan 2023 15:36:24 -0500
From:   Alan Stern <stern@rowland.harvard.edu>
To:     "Paul E. McKenney" <paulmck@kernel.org>
Cc:     Andrea Parri <parri.andrea@gmail.com>,
        Jonas Oberhauser <jonas.oberhauser@huawei.com>,
        Peter Zijlstra <peterz@infradead.org>, will <will@kernel.org>,
        "boqun.feng" <boqun.feng@gmail.com>, npiggin <npiggin@gmail.com>,
        dhowells <dhowells@redhat.com>,
        "j.alglave" <j.alglave@ucl.ac.uk>,
        "luc.maranget" <luc.maranget@inria.fr>, akiyks <akiyks@gmail.com>,
        dlustig <dlustig@nvidia.com>, joel <joel@joelfernandes.org>,
        urezki <urezki@gmail.com>,
        quic_neeraju <quic_neeraju@quicinc.com>,
        frederic <frederic@kernel.org>,
        Kernel development list <linux-kernel@vger.kernel.org>
Subject: Re: Internal vs. external barriers (was: Re: Interesting LKMM litmus
 test)
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On Fri, Jan 20, 2023 at 11:20:32AM -0800, Paul E. McKenney wrote:
> On Fri, Jan 20, 2023 at 01:37:51PM -0500, Alan Stern wrote:
> > srcu_read_unlock() does not need a full smp_mb().
> 
> That is quite possible, and that is what we are looking into.  And testing
> thus far agrees with you.  But the grace-period ordering constraints
> are quite severe, so this requires careful checking and severe testing.

If you're interested, I can provide a simple argument to show that the 
Fundamental Law of RCU would continue to hold with only a release fence.  
There is an added requirement: merely that synchronize_srcu() must have 
an smp_mb() somewhere after its final read of the unlock counters -- 
which your version of the algorithm already has.

Alan