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Subject: Re: [PATCH 03/10] hugetlb: Use LIST_HEAD() to define a list head
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From:   Muchun Song <muchun.song@linux.dev>
In-Reply-To: <bb05153c-29a9-b77e-0815-506309da6da0@huawei.com>
Date:   Sat, 27 Aug 2022 10:48:34 +0800
Cc:     Andrew Morton <akpm@linux-foundation.org>, mike.kravetz@oracle.com,
        Muchun Song <songmuchun@bytedance.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
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Message-Id: <83DBE074-0B63-4CB2-9BD2-65967977B69C@linux.dev>
References: <20220826092422.39591-1-linmiaohe@huawei.com>
 <20220826092422.39591-4-linmiaohe@huawei.com>
 <8D011A6F-F557-4519-852C-80BBDE53E630@linux.dev>
 <bb05153c-29a9-b77e-0815-506309da6da0@huawei.com>
To:     Miaohe Lin <linmiaohe@huawei.com>
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> On Aug 27, 2022, at 10:27, Miaohe Lin <linmiaohe@huawei.com> wrote:
>=20
> On 2022/8/27 9:47, Muchun Song wrote:
>>=20
>>=20
>>> On Aug 26, 2022, at 17:24, Miaohe Lin <linmiaohe@huawei.com> wrote:
>>>=20
>>> We can avoid unneeded WRITE_ONCE() overhead by using LIST_HEAD() to =
define
>>> a list head.
>>=20
>> IIUC, the overhead doesn=E2=80=99t change. Right?
>=20
> I think the overhead is changed. LIST_HEAD is initialized without =
using WRITE_ONCE():

I think there is no special difference with "WRITE_ONCE(var, 0)" vs "var =
=3D 0=E2=80=9D in
assembly code. Both code of line will be compiled to a mov or movq =
instruction.
I didn=E2=80=99t confirm if the assembly code is different (I tend to =
think it is similar).
Just some analysis from me.

>=20
> #define LIST_HEAD_INIT(name) { &(name), &(name) }
>=20
> #define LIST_HEAD(name) \
> 	struct list_head name =3D LIST_HEAD_INIT(name)
>=20
> while INIT_LIST_HEAD has:
>=20
> static inline void INIT_LIST_HEAD(struct list_head *list)
> {
> 	WRITE_ONCE(list->next, list);
> 	WRITE_ONCE(list->prev, list);
> }
>=20
> Or am I miss something?
>=20
>>=20
>> I=E2=80=99m fine with your changes.
>>=20
>> Reviewed-by: Muchun Song <songmuchun@bytedance.com>
>=20
> Many thanks for your review and comment. :)
>=20
> Thanks,
> Miaohe Lin
>=20