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:   Fri, 20 Jan 2023 13:20:37 -0800
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Alan Stern <stern@rowland.harvard.edu>
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)
Message-ID: <20230120212037.GW2948950@paulmck-ThinkPad-P17-Gen-1>
Reply-To: paulmck@kernel.org
References: <20230118035041.GQ2948950@paulmck-ThinkPad-P17-Gen-1>
 <Y8gjUKoHxqR9+7Hx@rowland.harvard.edu>
 <20230118200601.GH2948950@paulmck-ThinkPad-P17-Gen-1>
 <Y8hclxuhpGm+krkz@rowland.harvard.edu>
 <20230119000214.GM2948950@paulmck-ThinkPad-P17-Gen-1>
 <Y8q6v3BZ8dlyoTxo@rowland.harvard.edu>
 <20230120175804.GN2948950@paulmck-ThinkPad-P17-Gen-1>
 <Y8rff3HJ2o9wUyGT@rowland.harvard.edu>
 <20230120192032.GR2948950@paulmck-ThinkPad-P17-Gen-1>
 <Y8r7SBdfuZX/y1cd@rowland.harvard.edu>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <Y8r7SBdfuZX/y1cd@rowland.harvard.edu>
Precedence: bulk

On Fri, Jan 20, 2023 at 03:36:24PM -0500, Alan Stern wrote:
> 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.

Please!

For your amusement, here is a very informal argument that this is
the case:

https://docs.google.com/document/d/1xvwQzavmH474MBPAIBqVyvCrCcS5j2BpqhErPhRj7Is/edit?usp=sharing

See the "Read-Side Optimizations" section at the end.

							Thanx, Paul