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Date:   Tue, 31 May 2022 13:58:31 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Miaohe Lin <linmiaohe@huawei.com>
Cc:     akpm@linux-foundation.org, naoya.horiguchi@nec.com,
        peterx@redhat.com, apopple@nvidia.com, ying.huang@intel.com,
        osalvador@suse.de, mike.kravetz@oracle.com,
        songmuchun@bytedance.com, hch@lst.de, dhowells@redhat.com,
        cl@linux.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v4 1/4] mm: reduce the rcu lock duration
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References: <20220530113016.16663-1-linmiaohe@huawei.com>
 <20220530113016.16663-2-linmiaohe@huawei.com>
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On Mon, May 30, 2022 at 07:30:13PM +0800, Miaohe Lin wrote:
> Commit 3268c63eded4 ("mm: fix move/migrate_pages() race on task struct")
> extends the period of the rcu_read_lock until after the permissions checks
> are done to prevent the task pointed to from changing from under us. But
> the task_struct refcount is also taken at that time, the reference to task
> is guaranteed to be stable. So it's unnecessary to extend the period of
> the rcu_read_lock. Release the rcu lock after task refcount is successfully
> grabbed to reduce the rcu holding time.

But why bother?  You know the RCU read lock isn't a "real" lock, right?