linux-ext4 - Re: [PATCH 11/12] xfs: drop s_umount over opening the log and RT devices

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Date:   Sat, 5 Aug 2023 10:32:39 +0200
From:   Christoph Hellwig <hch@....de>
To:     "Darrick J. Wong" <djwong@...nel.org>
Cc:     Christoph Hellwig <hch@....de>, Al Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>,
        Jan Kara <jack@...e.cz>, Chris Mason <clm@...com>,
        Josef Bacik <josef@...icpanda.com>,
        David Sterba <dsterba@...e.com>, Theodore Ts'o <tytso@....edu>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Jaegeuk Kim <jaegeuk@...nel.org>, Chao Yu <chao@...nel.org>,
        Ryusuke Konishi <konishi.ryusuke@...il.com>,
        Jens Axboe <axboe@...nel.dk>, linux-btrfs@...r.kernel.org,
        linux-ext4@...r.kernel.org, linux-f2fs-devel@...ts.sourceforge.net,
        linux-nilfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-xfs@...r.kernel.org, linux-block@...r.kernel.org
Subject: Re: [PATCH 11/12] xfs: drop s_umount over opening the log and RT
 devices

On Wed, Aug 02, 2023 at 09:32:19AM -0700, Darrick J. Wong wrote:
> > +	/* 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);

Yes.  With the refactoring earlier in the series get_tree_bdev should
be trivial enough to not need a more specific reference.  If you
think there's a better way to refer to it I can update the comment,
though.

> >  		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.

Agreed.  Christian, can you just change this in your branch, or should
I send an incremental patch?