linux-kernel - Re: [RFC PATCH v2] memory-barriers: remove smp_mb__after_unlock

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: Windows password security audit tool. GUI, reports in PDF.

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

Message-ID: <1436827424.3948.239.camel@kernel.crashing.org>
Date:	Tue, 14 Jul 2015 08:43:44 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Peter Hurley <peter@...leysoftware.com>,
	Will Deacon <will.deacon@....com>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH v2] memory-barriers: remove
 smp_mb__after_unlock_lock()

On Tue, 2015-07-14 at 00:15 +0200, Peter Zijlstra wrote:

> > This is instead the sequence that is of concern:
> > 
> > 	store a
> > 	unlock M
> > 	lock N
> > 	load b
> 
> So its late and that table didn't parse, but that should be ordered too.
> The load of b should not be able to escape the lock N.
> 
> If only because LWSYNC is a valid RMB and any LOCK implementation must
> load the lock state to observe it unlocked.

What happens is that the load passes the store conditional, though it
doesn't pass the load with reserve. However, both store A and unlock M
being just stores with an lwsync, can pass a load, so they can pass the
load with reserve. And thus inside the LL/SC loop, our store A has
passed our load B.

> > > Additionally, the assertion in Documentation/memory_barriers.txt that
> > > the sequence above can be reordered as
> > > 
> > >   LOCK N
> > >   store b
> > >   store a
> > >   UNLOCK M
> > > 
> > > is not true on any existing arch in Linux.
> > 
> > It was at one time and might be again.
> 
> What would be required to make this true? I'm having a hard time seeing
> how things can get reordered like that.


--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/