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

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Date:   Fri, 13 Dec 2019 18:19:39 -0500
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Jan Kara <jack@...e.cz>
Cc:     Phong Tran <tranmanphong@...il.com>,
        "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 08:49:43PM +0100, Jan Kara wrote:
> On Fri 13-12-19 10:11:50, Joel Fernandes wrote:
> > 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?
> 
> Well, not really. The code is active only with CONFIG_EXT4_DEBUG enabled
> and in this case there's no race with remount (and thus sbi->system_blks
> changing) possible. So the change is really only to silence the sparse
> warning.

Ok, thanks for clarifying.

-Joel

> 
> 								Honza
> 
> > 
> > - 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
> > >
> -- 
> Jan Kara <jack@...e.com>
> SUSE Labs, CR