linux-kernel - Re: Inconsistent description in memory-barrier.txt

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Date:	Wed, 30 Dec 2015 10:18:44 +0800
From:	Jianyu Zhan <nasa4836@...il.com>
To:	Paul McKenney <paulmck@...ux.vnet.ibm.com>
Cc:	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Inconsistent description in memory-barrier.txt

On Wed, Dec 30, 2015 at 8:26 AM, Paul E. McKenney
<paulmck@...ux.vnet.ibm.com> wrote:
> It looks like you are quite correct, good catch!  Does the patch below
> fix this?
>
>                                                         Thanx, Paul
>
> ------------------------------------------------------------------------
>
> commit add179813efa2ba8a4afd29828d3335cf346d2a8
> Author: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> Date:   Tue Dec 29 16:23:18 2015 -0800
>
>     documentation: Fix control dependency and identical stores
>
>     The summary of the "CONTROL DEPENDENCIES" section incorrectly states that
>     barrier() may be used to prevent compiler reordering when more than one
>     leg of the control-dependent "if" statement start with identical stores.
>     This is incorrect at high optimization levels.  This commit therefore
>     updates the summary to match the detailed description.
>
>     Reported by: Jianyu Zhan <nasa4836@...il.com>
>     Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
>
> diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
> index 85304ebd187c..50190368400c 100644
> --- a/Documentation/memory-barriers.txt
> +++ b/Documentation/memory-barriers.txt
> @@ -800,9 +800,13 @@ In summary:
>        use smp_rmb(), smp_wmb(), or, in the case of prior stores and
>        later loads, smp_mb().
>
> -  (*) If both legs of the "if" statement begin with identical stores
> -      to the same variable, a barrier() statement is required at the
> -      beginning of each leg of the "if" statement.
> +  (*) If both legs of the "if" statement begin with identical stores to
> +      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.
>
>    (*) Control dependencies require at least one run-time conditional
>        between the prior load and the subsequent store, and this


Yep. It looks good to me.  Concise, precise wording, as always.


Thanks,
Jianyu Zhan
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