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

[James Bottomley <James.Bottomley@HansenPartnership.com>]

On Sun, 2017-01-08 at 15:52 +0100, Christoph Hellwig wrote:
> On Sun, Jan 08, 2017 at 09:48:44AM -0500, Mimi Zohar wrote:
> > IMA takes the i_rwsem (fomerly i_mutex) before reading the file to
> > synchronize calculating the file hash and validating the file's
> > hash/signature stored as security.ima xattr
> 
> Well, it shouldn't do that.  In the I/O path i_rwsem is up to the
> fs to use.  Various other file systems also take it internally for
> reads, although mostly only for direct I/O.

Hey, that's not really true: the inode lock (i_rwsem) is used in all
sorts of generic places, including generic_file_write_iter().  That's,
I think, why ima is using it to try to prevent writes while it measures
the file.

> So the answer here is that ima needs to stop playing with i_rwsem.

Isn't there a happy medium? most sensible filesystems will allow shared
reading (unless they want to tank performance) so we can rely on the
fact that even if a fs does use i_rwsem internally on the read path, it
will have to be shared.  So simply replacing the inode_lock() in ima
with inode_lock_shared() should do what ima wants and not interact
badly even if the underlying FS uses i_rwsem.  If there's ever a FS
that takes it exclusively in the read path, ima can simply blacklist
it.

James