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Message-ID: <e425c28d-c285-0fc6-b692-744df0bde3a8@gmail.com>
Date: Sun, 9 Apr 2023 09:11:58 +0900
MIME-Version: 1.0
Subject: Re: [PATCH] CodeSamples/count: Remove unnecessary memory barriers
References: <20230407175813.1334028-1-mmpgouride@gmail.com>
 <81ADBFE2-3576-48EC-A892-B49A5C265487@gmail.com>
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From: Akira Yokosawa <akiyks@gmail.com>
In-Reply-To: <81ADBFE2-3576-48EC-A892-B49A5C265487@gmail.com>
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To: Alan Huang <mmpgouride@gmail.com>, paulmck@kernel.org
Cc: perfbook@vger.kernel.org, Akira Yokosawa <akiyks@gmail.com>
List-ID: <perfbook.vger.kernel.org>

Hi,

On Sat, 8 Apr 2023 02:04:24 +0800, Alan Huang wrote:
> Hi Paul and Akira,
>=20
> This is the patch v3, I forgot to add a version tag=E2=80=A6
>=20
> And I think it may be better to add a question in a subsequent path.

So I'm looking at Quick Quizzes on count_lim_sig.c.
It looks to me Quick Quiz 5.50 lost its context due to the
removal of smp_mb() and the use of smp_store_release().

A band-aide fix would be to change the quiz to:

    In Listing 5.18's function flush_local_count_sig(), why
    are there READ_ONCE(), WRITE_ONCE(), and smp_store_release()
    wrappers around the uses of the theft per-thread variable?

, and to adjust the line count in its Answer.

However, smp_store_release() is by no means a simple wrapper.
So I'm wondering if it is worthwhile to keep this quiz.

Paul, how about replacing the quiz with the new quiz asking:

     In Listing 5.18, doesn't flush_local_count_sig() need
     a stronger memory barrier or two?

, and picking Alan's reasoning in the answer?

        Thanks, Akira

>=20
> Thanks,
> Alan