linux-kernel - Re: [PATCH] hugetlbfs: lockdep annotate root inode properly

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Date:	Thu, 8 Mar 2012 21:49:52 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Tyler Hicks <tyhicks@...onical.com>,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	linux-mm@...ck.org, davej@...hat.com, jboyer@...hat.com,
	linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Mimi Zohar <zohar@...ux.vnet.ibm.com>,
	David Gibson <david@...son.dropbear.id.au>
Subject: Re: [PATCH] hugetlbfs: lockdep annotate root inode properly

On Thu, Mar 08, 2012 at 01:40:50PM -0800, Andrew Morton wrote:

> OK, thanks, yup.  Taking i_mutex in file_operations.mmap() is wrong.

... or in .release() (munmap() does fput() under mmap_sem).

> Is hugetlbfs actually deadlockable because of this, or is it the case
> that the i_mutex->mmap_sem ordering happens to never happen for this
> filesystem?

Yes, it is.  Look at read(2) on hugetlbfs; it copies userland data
while holding ->i_mutex.  So we have

read(2):
mutex_lock(&A)
down_read(&B)

mmap(2):
down_write(&B);
mutex_lock(&A);

which is an obvious deadlock.

> So we need to pull the i_mutex out of hugetlbfs_file_mmap().

IIRC, you have a patch in your tree doing just that...
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