linux-kernel - Re: Litmus test for question from Al Viro

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Message-ID: <20201005154925.GY29330@paulmck-ThinkPad-P72>
Date:   Mon, 5 Oct 2020 08:49:25 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Alan Stern <stern@...land.harvard.edu>,
        Will Deacon <will@...nel.org>, parri.andrea@...il.com,
        boqun.feng@...il.com, npiggin@...il.com, dhowells@...hat.com,
        j.alglave@....ac.uk, luc.maranget@...ia.fr, akiyks@...il.com,
        dlustig@...dia.com, joel@...lfernandes.org,
        viro@...iv.linux.org.uk, linux-kernel@...r.kernel.org,
        linux-arch@...r.kernel.org
Subject: Re: Litmus test for question from Al Viro

On Mon, Oct 05, 2020 at 05:35:19PM +0200, Peter Zijlstra wrote:
> On Mon, Oct 05, 2020 at 11:16:39AM -0400, Alan Stern wrote:
> > On Mon, Oct 05, 2020 at 04:13:13PM +0100, Will Deacon wrote:
> > > > The failure to recognize the dependency in P0 should be considered a 
> > > > combined limitation of the memory model and herd7.  It's not a simple 
> > > > mistake that can be fixed by a small rewrite of herd7; rather it's a 
> > > > deliberate choice we made based on herd7's inherent design.  We 
> > > > explicitly said that control dependencies extend only to the code in the 
> > > > branches of an "if" statement; anything beyond the end of the statement 
> > > > is not considered to be dependent.
> > > 
> > > Interesting. How does this interact with loops that are conditionally broken
> > > out of, e.g.  a relaxed cmpxchg() loop or an smp_cond_load_relaxed() call
> > > prior to a WRITE_ONCE()?
> > 
> > Heh --  We finesse this issue by not supporting loops at all!  :-)
> 
> Right, so something like:
> 
> 	smp_cond_load_relaxed(x, !VAL);
> 	WRITE_ONCE(*y, 1);
> 
> Would be modeled like:
> 
> 	r1 = READ_ONCE(*x);
> 	if (!r1)
> 		WRITE_ONCE(*y, 1);
> 
> with an r1==0 constraint in the condition I suppose ?

Yes, you got it!

However, it is more efficient to use the "filter" clause to tell herd7
about executions that are to be discarded.

							Thanx, Paul