Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Tue, 27 Dec 2022 19:55:09 +0000
From:   Matthew Wilcox <willy@infradead.org>
To:     SeongJae Park <sj@kernel.org>
Cc:     akpm@linux-foundation.org, alexander.h.duyck@linux.intel.com,
        paulmck@kernel.org, linux-mm@kvack.org, rcu@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm/page_reporting: replace rcu_access_pointer() with
 rcu_dereference_protected()
Message-ID: <Y6tNnQvGGV+JVgC7@casper.infradead.org>
References: <20221227192158.2553-1-sj@kernel.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20221227192158.2553-1-sj@kernel.org>
Precedence: bulk

On Tue, Dec 27, 2022 at 07:21:58PM +0000, SeongJae Park wrote:
> +++ b/mm/page_reporting.c
> @@ -356,7 +356,7 @@ 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, true)) {

Pretty sure that passing a bare 'true' is an antipattern.
Instead, document _what_ lock protects us, ie:

	if (rcu_dereference_protected(pr_dev_info,
				      lockdep_is_held(&page_reporting_mutex))) {

Obviously, we took it just one line up, but if code moves around, it
may save us.