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

[Mimi Zohar <zohar@linux.vnet.ibm.com>]

On Sun, 2017-01-08 at 16:14 +0100, Christoph Hellwig wrote:
> 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.

We're only interested in the integrity of the local file system.

> > 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.

Christoph,  this isn't a new story and telling me this isn't very
productive.  Originally there was an IMA specific lock.  The i_mutex was
taken just to access the xattr.   Unforutnately, having two locks caused
a lockdep between the normal read/validate and setxattr.  As a result,
we dropped the IMA specific lock.

IMA needs a mechanism for quickly reading a file to calculate the file
hash and validate (or set) the file signature/hash stored as an xattr,
prior to any other process getting access to the file.

Mimi