linux-kernel - Re: [PATCH v3] tools/memory-model: Make ppo a subrelation of po

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Message-ID: <Y/rSQ2FNTJyj2bqR@rowland.harvard.edu>
Date:   Sat, 25 Feb 2023 22:30:11 -0500
From:   Alan Stern <stern@...land.harvard.edu>
To:     Boqun Feng <boqun.feng@...il.com>
Cc:     "Paul E. McKenney" <paulmck@...nel.org>,
        Jonas Oberhauser <jonas.oberhauser@...wei.com>,
        parri.andrea@...il.com, will@...nel.org, peterz@...radead.org,
        npiggin@...il.com, dhowells@...hat.com, j.alglave@....ac.uk,
        luc.maranget@...ia.fr, akiyks@...il.com, dlustig@...dia.com,
        joel@...lfernandes.org, urezki@...il.com, quic_neeraju@...cinc.com,
        frederic@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] tools/memory-model: Make ppo a subrelation of po

On Sat, Feb 25, 2023 at 07:09:05PM -0800, Boqun Feng wrote:
> On Sat, Feb 25, 2023 at 09:29:51PM -0500, Alan Stern wrote:
> > On Sat, Feb 25, 2023 at 05:01:10PM -0800, Paul E. McKenney wrote:
> > > A few other oddities:
> > > 
> > > litmus/auto/C-LB-Lww+R-OC.litmus
> > > 
> > > 	Both versions flag a data race, which I am not seeing.	It appears
> > > 	to me that P1's store to u0 cannot happen unless P0's store
> > > 	has completed.  So what am I missing here?
> > 
> > The LKMM doesn't believe that a control or data dependency orders a 
> > plain write after a marked read.  Hence in this test it thinks that P1's 
> > store to u0 can happen before the load of x1.  I don't remember why we 
> > did it this way -- probably we just wanted to minimize the restrictions 
> > on when plain accesses can execute.  (I do remember the reason for 
> > making address dependencies induce order; it was so RCU would work.)
> > 
> 
> Because plain store can be optimzed as an "store only if not equal"?
> As the following sentenses in the explanations.txt:
> 
> 	The need to distinguish between r- and w-bounding raises yet another
> 	issue.  When the source code contains a plain store, the compiler is
> 	allowed to put plain loads of the same location into the object code.
> 	For example, given the source code:
> 
> 		x = 1;
> 
> 	the compiler is theoretically allowed to generate object code that
> 	looks like:
> 
> 		if (x != 1)
> 			x = 1;
> 
> 	thereby adding a load (and possibly replacing the store entirely).
> 	For this reason, whenever the LKMM requires a plain store to be
> 	w-pre-bounded or w-post-bounded by a marked access, it also requires
> 	the store to be r-pre-bounded or r-post-bounded, so as to handle cases
> 	where the compiler adds a load.

Good guess; maybe that was the reason.  Ironically, in this case the 
store _is_ r-pre-bounded, because there's an smp_rmb() in the litmus 
test just before it.  So perhaps the original reason is not valid now 
that the memory model explicitly includes tests for stores being 
r-pre/post-bounded.

Alan