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Date:   Thu, 5 Apr 2018 09:32:04 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Alan Stern <stern@rowland.harvard.edu>
Cc:     Daniel Jordan <daniel.m.jordan@oracle.com>, parri.andrea@gmail.com,
        will.deacon@arm.com, boqun.feng@gmail.com, npiggin@gmail.com,
        dhowells@redhat.com, j.alglave@ucl.ac.uk, luc.maranget@inria.fr,
        paulmck@linux.vnet.ibm.com, akiyks@gmail.com,
        linux-kernel@vger.kernel.org,
        Steven Sistare <steven.sistare@oracle.com>,
        Pasha Tatashin <pasha.tatashin@oracle.com>
Subject: Re: Control dependency between prior load in while condition and
 later store?
Message-ID: <20180405073204.GP4043@hirez.programming.kicks-ass.net>
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 <Pine.LNX.4.44L0.1804041623530.1401-100000@iolanthe.rowland.org>
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On Wed, Apr 04, 2018 at 04:35:32PM -0400, Alan Stern wrote:
> On Wed, 4 Apr 2018, Daniel Jordan wrote:
> 
> > 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 would say that yes, there is.

Indeed.

> Yes, except that a more accurate view of the object code would be
> something like this:
> 
> Loop:	r1 = READ_ONCE(a);
> 	if (r1)
> 		goto Loop;
> 	else
> 		;	// Do nothing
> 	WRITE_ONCE(c, 1);
> 
> Here you can see that one path branches backward, so everything 
> following the "if" is dependent on the READ_ONCE.

Agreed, and I think I even have code that relies on such a pattern
somewhere.. Ah.. yes, see smp_cond_load_acquire().