Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Fri, 15 Oct 2021 06:19:18 -0700
From:   Christoph Hellwig <hch@infradead.org>
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Zqiang <qiang.zhang1211@gmail.com>, hch@infradead.org,
        akpm@linux-foundation.org, sunhao.th@gmail.com,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Mikulas Patocka <mpatocka@redhat.com>,
        Jens Axboe <axboe@kernel.dk>, Tejun Heo <tj@kernel.org>
Subject: Re: [PATCH] mm: backing-dev: use kfree_rcu() instead of
 synchronize_rcu_expedited()
Message-ID: <YWl/1gI+O2+PnKhz@infradead.org>
References: <20211014082433.30733-1-qiang.zhang1211@gmail.com>
 <YWgTZjDtZik/9l4J@casper.infradead.org>
 <CALm+0cUt7iD5zex4-hRv=i1wPd66tz3JYGHz8P8ZFTZcyOCD1A@mail.gmail.com>
 <d697d61e-27a2-a25c-3ae1-e41457d08705@gmail.com>
 <YWl1rDO6gCFJE4hp@casper.infradead.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <YWl1rDO6gCFJE4hp@casper.infradead.org>
Precedence: bulk

On Fri, Oct 15, 2021 at 01:35:56PM +0100, Matthew Wilcox wrote:
>  struct backing_dev_info {
>         u64 id;
> -       struct rb_node rb_node; /* keyed by ->id */
> +       union {
> +               struct rb_node rb_node; /* keyed by ->id */
> +               struct rcu_head rcu;
> +       };
>         struct list_head bdi_list;
>         unsigned long ra_pages; /* max readahead in PAGE_SIZE units */
>         unsigned long io_pages; /* max allowed IO size */
> 
> 
> Christoph, independent of the inode lifetime problem, this actually seems
> like a good approach to take.  I don't see why we should synchronize_rcu()
> here?  Adding Jens (original introducer of the synchronize_rcu()), Mikulas
> (converted it to use _expedited) and Tejun (worked around a problem when
> using _expedited).

The kfree+rcu + your suggestion does seem like a good idea in general to
me.  But I'd still like to fix the actual bug being reported before
optimizing the area in a way that papers over the bug.