linux-kernel - Re: [PATCH memory-barriers.txt 1/7] documentation: Clarify relationship of barrier() to control dependencies

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: <20160413235614.3517e030@grimm.local.home>
Date:	Wed, 13 Apr 2016 23:56:14 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org,
	jiangshanlai@...il.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
	dhowells@...hat.com, edumazet@...gle.com, dvhart@...ux.intel.com,
	fweisbec@...il.com, oleg@...hat.com, bobby.prani@...il.com
Subject: Re: [PATCH memory-barriers.txt 1/7] documentation: Clarify
 relationship of barrier() to control dependencies

On Tue, 12 Apr 2016 08:52:49 -0700
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:

> The current documentation claims that the compiler ignores barrier(),
> which is not the case.  Instead, the compiler carefully pays attention
> to barrier(), but in a creative way that still manages to destroy
> the control dependency.  This commit sets the story straight.
> 
> Reported-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> ---
>  Documentation/memory-barriers.txt | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
> index 3729cbe60e41..ec1289042396 100644
> --- a/Documentation/memory-barriers.txt
> +++ b/Documentation/memory-barriers.txt
> @@ -813,9 +813,10 @@ In summary:
>        the same variable, then those stores must be ordered, either by
>        preceding both of them with smp_mb() or by using smp_store_release()
>        to carry out the stores.  Please note that it is -not- sufficient
> -      to use barrier() at beginning of each leg of the "if" statement,
> -      as optimizing compilers do not necessarily respect barrier()
> -      in this case.
> +      to use barrier() at beginning of each leg of the "if" statement
> +      because, as shown by the example above, optimizing compilers can
> +      destroy the control dependency while respecting the letter of the
> +      barrier() law.

Which country has the jurisdiction over this barrier() law?

What about "the letter of the barrier() rules"?

-- Steve

>  
>    (*) Control dependencies require at least one run-time conditional
>        between the prior load and the subsequent store, and this