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Subject: Re: [PATCH] fs: btrfs: block-group.c: Fix suspicious RCU usage
 warning
To:     madhuparnabhowmik10@gmail.com, clm@fb.com, josef@toxicpanda.com,
        dsterba@suse.com
Cc:     linux-btrfs@vger.kernel.org, linux-kernel@vger.kernel.org,
        joel@joelfernandes.org,
        linux-kernel-mentees@lists.linuxfoundation.org, paulmck@kernel.org,
        frextrite@gmail.com, linux@roeck-us.net
References: <20200306065243.11699-1-madhuparnabhowmik10@gmail.com>
From:   Qu Wenruo <quwenruo.btrfs@gmx.com>
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Message-ID: <dfd2c14c-acda-3862-9f48-a512e16a895c@gmx.com>
Date:   Fri, 6 Mar 2020 15:16:53 +0800
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On 2020/3/6 =E4=B8=8B=E5=8D=882:52, madhuparnabhowmik10@gmail.com wrote:
> From: Madhuparna Bhowmik <madhuparnabhowmik10@gmail.com>
>=20
> The space_info list is rcu protected.
> Hence, it should be traversed with rcu_read_lock held.
>=20
> Warning:
> [   29.104591] =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
> [   29.104756] WARNING: suspicious RCU usage
> [   29.105046] 5.6.0-rc4-next-20200305 #1 Not tainted
> [   29.105231] -----------------------------
> [   29.105401] fs/btrfs/block-group.c:2011 RCU-list traversed in non-re=
ader section!!
>=20
> Reported-by: Guenter Roeck <linux@roeck-us.net>
> Signed-off-by: Madhuparna Bhowmik <madhuparnabhowmik10@gmail.com>
> ---
>  fs/btrfs/block-group.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>=20
> diff --git a/fs/btrfs/block-group.c b/fs/btrfs/block-group.c
> index 404e050ce8ee..9cabeef66f5b 100644
> --- a/fs/btrfs/block-group.c
> +++ b/fs/btrfs/block-group.c
> @@ -1987,6 +1987,7 @@ int btrfs_read_block_groups(struct btrfs_fs_info =
*info)

This function is only triggered at mount time, where no other rcu
operation can happen.

Thanks,
Qu
>  		btrfs_release_path(path);
>  	}
> =20
> +	rcu_read_lock();
>  	list_for_each_entry_rcu(space_info, &info->space_info, list) {
>  		if (!(btrfs_get_alloc_profile(info, space_info->flags) &
>  		      (BTRFS_BLOCK_GROUP_RAID10 |
> @@ -2007,7 +2008,8 @@ int btrfs_read_block_groups(struct btrfs_fs_info =
*info)
>  				list)
>  			inc_block_group_ro(cache, 1);
>  	}
> -
> +	rcu_read_unlock();
> +	=09
>  	btrfs_init_global_block_rsv(info);
>  	ret =3D check_chunk_block_group_mappings(info);
>  error:
>=20


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