Thanks Paul,
On 10/31/2017 11:50 AM, Paul E. McKenney wrote:
> On Tue, Oct 31, 2017 at 11:14:47AM +0800, Yubin Ruan wrote:
>> >From 256adc5ccd239288c3f38cd193072d6666ab1e82 Mon Sep 17 00:00:00 2001
>> From: Yubin Ruan <ablacktshirt@xxxxxxxxx>
>> Date: Tue, 31 Oct 2017 11:12:03 +0800
>> Subject: [PATCH] memorder: Add one solution for one snippet
>>
>> Signed-off-by: Yubin Ruan <ablacktshirt@xxxxxxxxx>
>> ---
>> memorder/memorder.tex | 2 ++
>> 1 file changed, 2 insertions(+)
>>
>> diff --git a/memorder/memorder.tex b/memorder/memorder.tex
>> index f8886b4..a1c96df 100644
>> --- a/memorder/memorder.tex
>> +++ b/memorder/memorder.tex
>> @@ -3055,6 +3055,8 @@ surprise to \co{do_something()}.\footnote{
>> Tracking down the bug consumed a holiday weekend not just
>> for your editor, but also for several of his colleagues.
>> In short, this is not a new problem.}
>> +In this case, \co{tmp} should be declared as \co{volatile} to prevent
>> +the transformation by the compiler.
>>
>> Compilers can also fuse stores.
>> The most infamous example is probably the progress-bar example
>
> Declaring tmp as volatile could in some sense solve the problem, but at
> the expense of preventing the compiler from caching tmp in a register.
> It is better to use READ_ONCE() on the initial read from p, or failing
> that, to declare p (not tmp) volatile.
>
> Thanx, Paul
Acked-by: Yubin Ruan <ablacktshirt@xxxxxxxxx>
> ------------------------------------------------------------------------
>
> commit d1f08db32120f079da699c9deebde22af96a202a
> Author: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> Date: Mon Oct 30 20:44:33 2017 -0700
>
> memorder: Show how to use READ_ONCE() to prevent load replication
>
> Reported-by: Yubin Ruan <ablacktshirt@xxxxxxxxx>
> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
>
> diff --git a/memorder/memorder.tex b/memorder/memorder.tex
> index f8886b42468c..a3b8f7c86946 100644
> --- a/memorder/memorder.tex
> +++ b/memorder/memorder.tex
> @@ -3055,6 +3055,21 @@ surprise to \co{do_something()}.\footnote{
> Tracking down the bug consumed a holiday weekend not just
> for your editor, but also for several of his colleagues.
> In short, this is not a new problem.}
> +To prevent the compiler from replicating the load, use \co{READ_ONCE()},
> +for example as follows:
> +
> +\vspace{5pt}
> +\begin{minipage}[t]{\columnwidth}
> +\scriptsize
> +\begin{verbatim}
> + 1 tmp = READ_ONCE(p);
> + 2 if (tmp != NULL && tmp <= q)
> + 3 do_something(tmp);
> +\end{verbatim}
> +\end{minipage}
> +\vspace{5pt}
> +
> +Alternatively, the variable \co{p} could be declared \co{volatile}.
>
> Compilers can also fuse stores.
> The most infamous example is probably the progress-bar example
>
--
To unsubscribe from this list: send the line "unsubscribe perfbook" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
