linux-kernel - Re: xfs: commit 6552321831dc "xfs: remove i_iolock and use i_rwsem in the VFS inode instead" change causes hang

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Date:   Sun, 8 Jan 2017 16:14:30 +0100
From:   Christoph Hellwig <hch@....de>
To:     Mimi Zohar <zohar@...ux.vnet.ibm.com>
Cc:     Christoph Hellwig <hch@....de>, linux-xfs@...r.kernel.org,
        Dave Chinner <david@...morbit.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Al Viro <viro@...IV.linux.org.uk>
Subject: Re: xfs:  commit 6552321831dc "xfs: remove i_iolock and use
        i_rwsem in the VFS inode instead"  change causes hang

On Sun, Jan 08, 2017 at 10:03:09AM -0500, Mimi Zohar wrote:
> But not normally for a normal file read.

Depends on the file system.  In addition to XFS at least the NFS
also uses i_rwsem by default.  Also all file systems supporting
a DAX I/O path.

> Unless something has changed recently, to synchronize reading files to
> calculate the file hash and writing xattrs it has to take the i_rwsem
> prior to reading the file.

No, you must simply not do this at all.  If you take a lock that
belongs to the fs and is not your own over ->read_iter you're toast
as you've seen.