Date:   Tue, 24 Jan 2023 11:03:49 -0500
From:   Alan Stern <stern@rowland.harvard.edu>
To:     Jonas Oberhauser <jonas.oberhauser@huaweicloud.com>
Cc:     paulmck@kernel.org, parri.andrea@gmail.com, will@kernel.org,
        peterz@infradead.org, boqun.feng@gmail.com, npiggin@gmail.com,
        dhowells@redhat.com, j.alglave@ucl.ac.uk, luc.maranget@inria.fr,
        akiyks@gmail.com, dlustig@nvidia.com, joel@joelfernandes.org,
        urezki@gmail.com, quic_neeraju@quicinc.com, frederic@kernel.org,
        linux-kernel@vger.kernel.org, viktor@mpi-sws.org
Subject: Re: [PATCH] tools/memory-model: Make ppo a subrelation of po
Message-ID: <Y9ABZQ81oXTmggcY@rowland.harvard.edu>
References: <20230117193159.22816-1-jonas.oberhauser@huaweicloud.com>
 <Y8hN7vs/w8LhMasT@rowland.harvard.edu>
 <c22ec058-b058-0b6e-718b-348ff5cb5004@huaweicloud.com>
 <Y8i1QNjnZwim5uMq@rowland.harvard.edu>
 <1180fe22-5e1d-ec8b-8012-b6578b1ca7c0@huaweicloud.com>
 <Y87tHNcvb5E+t3da@rowland.harvard.edu>
 <6f8575f3-f8b9-7738-24f0-5e390b50ac40@huaweicloud.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=iso-8859-1
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
In-Reply-To: <6f8575f3-f8b9-7738-24f0-5e390b50ac40@huaweicloud.com>
Precedence: bulk

On Tue, Jan 24, 2023 at 01:54:14PM +0100, Jonas Oberhauser wrote:
> 
> 
> On 1/23/2023 9:25 PM, Alan Stern wrote:
> > On Mon, Jan 23, 2023 at 07:25:48PM +0100, Jonas Oberhauser wrote:
> > > Alright, after some synchronization in the other parts of this thread I am
> > > beginning to prepare the next iteration of the patch.
> > > 
> > > On 1/19/2023 4:13 AM, Alan Stern wrote:
> > > > On Wed, Jan 18, 2023 at 10:38:11PM +0100, Jonas Oberhauser wrote:
> > > > > On 1/18/2023 8:52 PM, Alan Stern wrote:
> > > > > > On Tue, Jan 17, 2023 at 08:31:59PM +0100, Jonas Oberhauser wrote:
> > > > > > > -	([M] ; po? ; [LKW] ; fencerel(After-spinlock) ; [M]) |
> > > > > > > -	([M] ; po ; [UL] ; (co | po) ; [LKW] ;
> > > > > > > -		fencerel(After-unlock-lock) ; [M])
> > > > > > > +	([M] ; po? ; [LKW] ; fencerel(After-spinlock) ; [M])
> > > > > > Shouldn't the po case of (co | po) remain intact here?
> > > > > You can leave it here, but it is already covered by two other parts: the
> > > > > ordering given through ppo/hb is covered by the po-unlock-lock-po & int in
> > > > > ppo, and the ordering given through pb is covered by its inclusion in
> > > > > strong-order.
> > > > What about the ordering given through
> > > > A-cumul(strong-fence)/cumul-fence/prop/hb?  I suppose that might be
> > > > superseded by pb as well, but it seems odd not to have it in hb.
> > > How should we resolve this?
> > > My current favorite (compromise :D) solution would be to
> > > 1. still eliminate both po and co cases from first definition of
> > > strong-fence which is used in ppo,
> > > 2. define a relation equal to the strong-order in this patch (with po|rf)
> > Wouldn't it need to have po|co?  Consider:
> > 
> > 	Wx=1	Rx=1		Ry=1		Rz=1
> > 		lock(s)		lock(s)		lock(s)
> > 		unlock(s)	unlock(s)	unlock(s)
> > 		Wy=1		Wz=1		smp_mb__after_unlock_lock
> > 						Rx=0
> > 
> > With the co term this is forbidden.  With only the rf term it is
> > allowed, because po-unlock-lock-po isn't A-cumulative.
> No, but unlock() is ( https://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git/tree/tools/memory-model/lock.cat?h=dev.2023.01.19a#n67
> ). So you get

So it is.  I had forgotten about that.  The model is getting too 
complicated to fit entirely in my mind...

> ? Rx=0 ->overwrite Wx=1? ->rfe Rx1 ->po-rel? T1:unlock(s) ->rfe T2:lock(s)
> ->po-unlock-lock-po;after ... fence;po Rx=0
> which is
> ? Rx=0 ???????? ->prop ;?????????????????????????? po-unlock-lock-po;after
> ... fence;po Rx=0
> 
> Are you ok going forward like this then?

I guess so, provided we mention somewhere in the code or documentation 
that this relation extends beyond a single rf.

Alan