linux-kernel - Re: [PATCH v2 17/26] selftests/resctrl: Replace file write with volatile variable

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Message-ID: <6358c720-66a9-4727-a74d-b56100f01ac4@intel.com>
Date:   Tue, 28 Nov 2023 14:16:28 -0800
From:   Reinette Chatre <reinette.chatre@...el.com>
To:     Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>,
        <linux-kselftest@...r.kernel.org>, Shuah Khan <shuah@...nel.org>,
        "Shaopeng Tan" <tan.shaopeng@...fujitsu.com>,
        Maciej Wieczór-Retman 
        <maciej.wieczor-retman@...el.com>,
        Fenghua Yu <fenghua.yu@...el.com>
CC:     <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 17/26] selftests/resctrl: Replace file write with
 volatile variable

Hi Ilpo,

On 11/20/2023 3:13 AM, Ilpo Järvinen wrote:
> The fill_buf code prevents compiler optimizating the entire read loop
> away by writing the final value of the variable into a file. While it
> achieves the goal, writing into a file requires significant amount of
> work within the innermost test loop and also error handling.
> 
> A simpler approach is to take advantage of volatile. Writing to a
> variable through a volatile pointer is enough to prevent compiler from
> optimizing the write away, and therefore compiler cannot remove the
> read loop either.
> 
> Add a volatile 'value_sink' into resctrl_tests.c and make fill_buf to
> write into it. As a result, the error handling in fill_buf.c can be
> simplified.
> 

The subject and changelog describes the need for a volatile variable.
The patch introduces two volatile variables. Could you please elaborate
why two volatile variables are needed?

Reinette