Re: [PATCH 7/7] md: fix a suspicious RCU usage warning

From: Song Liu <song@kernel.org>
To: Mikulas Patocka <mpatocka@redhat.com>
Cc: Yu Kuai <yukuai3@huawei.com>, David Jeffery <djeffery@redhat.com>,
	 Li Nan <linan122@huawei.com>,
	dm-devel@lists.linux.dev, linux-raid@vger.kernel.org,
	 Mike Snitzer <msnitzer@redhat.com>,
	Heinz Mauelshagen <heinzm@redhat.com>,
	 Benjamin Marzinski <bmarzins@redhat.com>
Subject: Re: [PATCH 7/7] md: fix a suspicious RCU usage warning
Date: Wed, 17 Jan 2024 15:59:54 -0800	[thread overview]
Message-ID: <CAPhsuW42XBw4nUWzssSMh7i=ULL++yF5twFB8d0digmVLXQ6AA@mail.gmail.com> (raw)
In-Reply-To: <51539879-e1ca-fde3-b8b4-8934ddedcbc@redhat.com>

On Wed, Jan 17, 2024 at 10:22 AM Mikulas Patocka <mpatocka@redhat.com> wrote:
>
> RCU protection was removed in the commit 2d32777d60de ("raid1: remove rcu
> protection to access rdev from conf").
>
> However, the code in fix_read_error does rcu_dereference outside
> rcu_read_lock - this triggers the following warning. The warning is
> triggered by a LVM2 test shell/integrity-caching.sh.
>
> This commit removes rcu_dereference.
>
> =============================
> WARNING: suspicious RCU usage
> 6.7.0 #2 Not tainted
> -----------------------------
> drivers/md/raid1.c:2265 suspicious rcu_dereference_check() usage!
>
> other info that might help us debug this:
>
> rcu_scheduler_active = 2, debug_locks = 1
> no locks held by mdX_raid1/1859.
>
> stack backtrace:
> CPU: 2 PID: 1859 Comm: mdX_raid1 Not tainted 6.7.0 #2
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-1 04/01/2014
> Call Trace:
>  <TASK>
>  dump_stack_lvl+0x60/0x70
>  lockdep_rcu_suspicious+0x153/0x1b0
>  raid1d+0x1732/0x1750 [raid1]
>  ? lock_acquire+0x9f/0x270
>  ? finish_wait+0x3d/0x80
>  ? md_thread+0xf7/0x130 [md_mod]
>  ? lock_release+0xaa/0x230
>  ? md_register_thread+0xd0/0xd0 [md_mod]
>  md_thread+0xa0/0x130 [md_mod]
>  ? housekeeping_test_cpu+0x30/0x30
>  kthread+0xdc/0x110
>  ? kthread_complete_and_exit+0x20/0x20
>  ret_from_fork+0x28/0x40
>  ? kthread_complete_and_exit+0x20/0x20
>  ret_from_fork_asm+0x11/0x20
>  </TASK>
>
> Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
> Fixes: ca294b34aaf3 ("md/raid1: support read error check")

This makes sense to me.

Li Nan, please review this fix.

Thanks,
Song

>
> ---
>  drivers/md/raid1.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> Index: linux-2.6/drivers/md/raid1.c
> ===================================================================
> --- linux-2.6.orig/drivers/md/raid1.c
> +++ linux-2.6/drivers/md/raid1.c
> @@ -2262,7 +2262,7 @@ static void fix_read_error(struct r1conf
>         int sectors = r1_bio->sectors;
>         int read_disk = r1_bio->read_disk;
>         struct mddev *mddev = conf->mddev;
> -       struct md_rdev *rdev = rcu_dereference(conf->mirrors[read_disk].rdev);
> +       struct md_rdev *rdev = conf->mirrors[read_disk].rdev;
>
>         if (exceed_read_errors(mddev, rdev)) {
>                 r1_bio->bios[r1_bio->read_disk] = IO_BLOCKED;
>