linux-ext4 - Re: [PATCH V2] ext4: use rcu API in debug_print

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Date:   Fri, 13 Dec 2019 10:11:50 -0800
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Phong Tran <tranmanphong@...il.com>
Cc:     Jan Kara <jack@...e.cz>, "Theodore Y. Ts'o" <tytso@....edu>,
        adilger.kernel@...ger.ca, "Paul E. McKenney" <paulmck@...nel.org>,
        linux-ext4@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        rcu <rcu@...r.kernel.org>, stable <stable@...r.kernel.org>
Subject: Re: [PATCH V2] ext4: use rcu API in debug_print_tree

On Fri, Dec 13, 2019 at 7:39 AM Phong Tran <tranmanphong@...il.com> wrote:
>
> struct ext4_sb_info.system_blks was marked __rcu.
> But access the pointer without using RCU lock and dereference.
> Sparse warning with __rcu notation:
>
> block_validity.c:139:29: warning: incorrect type in argument 1 (different address spaces)
> block_validity.c:139:29:    expected struct rb_root const *
> block_validity.c:139:29:    got struct rb_root [noderef] <asn:4> *
>
> Reviewed-by: Jan Kara <jack@...e.cz>
> Signed-off-by: Phong Tran <tranmanphong@...il.com>

Thanks Phong! Looks like a real bug fix caught thanks to Sparse. So
let us mark for stable as well?

- Joel

> ---
>  fs/ext4/block_validity.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
>
> ---
> change log:
> V2: Add Reviewed-by: Jan Kara <jack@...e.cz>
>
> diff --git a/fs/ext4/block_validity.c b/fs/ext4/block_validity.c
> index d4d4fdfac1a6..1ee04e76bbe0 100644
> --- a/fs/ext4/block_validity.c
> +++ b/fs/ext4/block_validity.c
> @@ -133,10 +133,13 @@ static void debug_print_tree(struct ext4_sb_info *sbi)
>  {
>         struct rb_node *node;
>         struct ext4_system_zone *entry;
> +       struct ext4_system_blocks *system_blks;
>         int first = 1;
>
>         printk(KERN_INFO "System zones: ");
> -       node = rb_first(&sbi->system_blks->root);
> +       rcu_read_lock();
> +       system_blks = rcu_dereference(sbi->system_blks);
> +       node = rb_first(&system_blks->root);
>         while (node) {
>                 entry = rb_entry(node, struct ext4_system_zone, node);
>                 printk(KERN_CONT "%s%llu-%llu", first ? "" : ", ",
> @@ -144,6 +147,7 @@ static void debug_print_tree(struct ext4_sb_info *sbi)
>                 first = 0;
>                 node = rb_next(node);
>         }
> +       rcu_read_unlock();
>         printk(KERN_CONT "\n");
>  }
>
> --
> 2.20.1
>