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Date:   Wed, 2 Aug 2023 09:32:19 -0700
From:   "Darrick J. Wong" <djwong@kernel.org>
To:     Christoph Hellwig <hch@lst.de>
Cc:     Al Viro <viro@zeniv.linux.org.uk>,
        Christian Brauner <brauner@kernel.org>,
        Jan Kara <jack@suse.cz>, Chris Mason <clm@fb.com>,
        Josef Bacik <josef@toxicpanda.com>,
        David Sterba <dsterba@suse.com>, Theodore Ts'o <tytso@mit.edu>,
        Andreas Dilger <adilger.kernel@dilger.ca>,
        Jaegeuk Kim <jaegeuk@kernel.org>, Chao Yu <chao@kernel.org>,
        Ryusuke Konishi <konishi.ryusuke@gmail.com>,
        Jens Axboe <axboe@kernel.dk>, linux-btrfs@vger.kernel.org,
        linux-ext4@vger.kernel.org, linux-f2fs-devel@lists.sourceforge.net,
        linux-nilfs@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-xfs@vger.kernel.org, linux-block@vger.kernel.org
Subject: Re: [PATCH 11/12] xfs: drop s_umount over opening the log and RT
 devices
Message-ID: <20230802163219.GW11352@frogsfrogsfrogs>
References: <20230802154131.2221419-1-hch@lst.de>
 <20230802154131.2221419-12-hch@lst.de>
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On Wed, Aug 02, 2023 at 05:41:30PM +0200, Christoph Hellwig wrote:
> Just like get_tree_bdev needs to drop s_umount when opening the main
> device, we need to do the same for the xfs log and RT devices to avoid a
> potential lock order reversal with s_unmount for the mark_dead path.
> 
> It might be preferable to just drop s_umount over ->fill_super entirely,
> but that will require a fairly massive audit first, so we'll do the easy
> version here first.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
>  fs/xfs/xfs_super.c | 15 +++++++++++----
>  1 file changed, 11 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
> index 8185102431301d..d5042419ed9997 100644
> --- a/fs/xfs/xfs_super.c
> +++ b/fs/xfs/xfs_super.c
> @@ -448,17 +448,21 @@ STATIC int
>  xfs_open_devices(
>  	struct xfs_mount	*mp)
>  {
> -	struct block_device	*ddev = mp->m_super->s_bdev;
> +	struct super_block	*sb = mp->m_super;
> +	struct block_device	*ddev = sb->s_bdev;
>  	struct block_device	*logdev = NULL, *rtdev = NULL;
>  	int			error;
>  
> +	/* see get_tree_bdev why this is needed and safe */

Which part of get_tree_bdev?  Is it this?

		/*
		 * s_umount nests inside open_mutex during
		 * __invalidate_device().  blkdev_put() acquires
		 * open_mutex and can't be called under s_umount.  Drop
		 * s_umount temporarily.  This is safe as we're
		 * holding an active reference.
		 */
		up_write(&s->s_umount);
		blkdev_put(bdev, fc->fs_type);
		down_write(&s->s_umount);

<confused>

> +	up_write(&sb->s_umount);
> +
>  	/*
>  	 * Open real time and log devices - order is important.
>  	 */
>  	if (mp->m_logname) {
>  		error = xfs_blkdev_get(mp, mp->m_logname, &logdev);
>  		if (error)
> -			return error;
> +			goto out_unlock;
>  	}
>  
>  	if (mp->m_rtname) {
> @@ -496,7 +500,10 @@ xfs_open_devices(
>  		mp->m_logdev_targp = mp->m_ddev_targp;
>  	}
>  
> -	return 0;
> +	error = 0;
> +out_unlock:
> +	down_write(&sb->s_umount);

Isn't down_write taking s_umount?  I think the label should be
out_relock or something less misleading.

--D

> +	return error;
>  
>   out_free_rtdev_targ:
>  	if (mp->m_rtdev_targp)
> @@ -508,7 +515,7 @@ xfs_open_devices(
>   out_close_logdev:
>  	if (logdev && logdev != ddev)
>  		xfs_blkdev_put(mp, logdev);
> -	return error;
> +	goto out_unlock;
>  }
>  
>  /*
> -- 
> 2.39.2
>