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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20180405073204.GP4043@hirez.programming.kicks-ass.net>
Date:   Thu, 5 Apr 2018 09:32:04 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     Daniel Jordan <daniel.m.jordan@...cle.com>, parri.andrea@...il.com,
        will.deacon@....com, 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: Re: Control dependency between prior load in while condition and
 later store?

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().