linux-kernel - Control dependency between prior load in while condition and later store?

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Date:   Wed, 4 Apr 2018 15:29:33 -0400
From:   Daniel Jordan <daniel.m.jordan@...cle.com>
To:     stern@...land.harvard.edu, parri.andrea@...il.com,
        will.deacon@....com, peterz@...radead.org, boqun.feng@...il.com,
        npiggin@...il.com, dhowells@...hat.com, j.alglave@....ac.uk,
        luc.maranget@...ia.fr, paulmck@...ux.vnet.ibm.com,
        akiyks@...il.com, linux-kernel@...r.kernel.org,
        Steven Sistare <steven.sistare@...cle.com>,
        Pasha Tatashin <pasha.tatashin@...cle.com>
Subject: Control dependency between prior load in while condition and later
 store?

A question for memory-barriers.txt aficionados.

Is there a control dependency between the prior load of 'a' and the 
later store of 'c'?:

   while (READ_ONCE(a));
   WRITE_ONCE(c, 1);

I have my doubts because memory-barriers.txt doesn't talk much about 
loops and because of what that document says here:

   In addition, control dependencies apply only to the then-clause and
   else-clause of the if-statement in question.  In particular, it does
   not necessarily apply to code following the if-statement:

   	q = READ_ONCE(a);
   	if (q) {
   		WRITE_ONCE(b, 1);
   	} else {
   		WRITE_ONCE(b, 2);
   	}
   	WRITE_ONCE(c, 1);  /* BUG: No ordering against the read from 'a'. */

It's not obvious to me how the then-clause/else-clause idea maps onto 
loops, but if we think of the example at the top like this...

   while (1) {
       if (!READ_ONCE(a)) {
           WRITE_ONCE(c, 1);
           break;
       }
   }

...then the dependent store is within the then-clause.  Viewed this way, 
it seems there would be a control dependency between a and c.

Is that right?

Thanks,
Daniel