linux-kernel - Re: [PATCH v2] mm/page_reporting: replace rcu_access_pointer() with rcu_dereference

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Message-ID: <4bc4ab74-3ccd-f892-b387-d48451463d3c@huawei.com>
Date:   Wed, 28 Dec 2022 09:45:00 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     SeongJae Park <sj@...nel.org>, <akpm@...ux-foundation.org>
CC:     <alexander.h.duyck@...ux.intel.com>, <paulmck@...nel.org>,
        <linux-mm@...ck.org>, <rcu@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] mm/page_reporting: replace rcu_access_pointer() with
 rcu_dereference_protected()

On 2022/12/28 9:29, SeongJae Park wrote:
> Page reporting fetches pr_dev_info using rcu_access_pointer(), which is
> for safely fetching a pointer that will not be dereferenced but could
> concurrently updated.  The code indeed does not dereference pr_dev_info
> after fetcing it using rcu_access_pointer(), but it fetches the pointer

Thanks for your work. Might something to improve.

s/fetcing/fetching/

> while concurrent updtes to the pointer is avoided by holding the update

s/updtes/updates/

> side lock, page_reporting_mutex.
> 
> In the case, rcu_dereference_protected() is recommended because it
> provides better readability and performance on some cases, as
> rcu_dereference_protected() avoids use of READ_ONCE().  Replace the
> rcu_access_pointer() calls with rcu_dereference_protected().
> 
> Signed-off-by: SeongJae Park <sj@...nel.org>
> ---
> Changes from v1
> (https://lore.kernel.org/linux-mm/20221227192158.2553-1-sj@kernel.org/)
> - Explicitly set the protection condition (Matthew Wilcox)
> 
>  mm/page_reporting.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/page_reporting.c b/mm/page_reporting.c
> index 79a8554f024c..5c557a3e1423 100644
> --- a/mm/page_reporting.c
> +++ b/mm/page_reporting.c
> @@ -356,7 +356,8 @@ int page_reporting_register(struct page_reporting_dev_info *prdev)
>  	mutex_lock(&page_reporting_mutex);
>  
>  	/* nothing to do if already in use */
> -	if (rcu_access_pointer(pr_dev_info)) {
> +	if (rcu_dereference_protected(pr_dev_info,
> +				lockdep_is_held(&page_reporting_order))) {

I think it should be lockdep_is_held(&page_reporting_mutex) instead of lockdep_is_held(&page_reporting_order) here?

Thanks,
Miaohe Lin