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SubjectRe: [PATCH] hugetlbfs: Add new rw_semaphore to fix truncate/read race
On Mon, 27 Feb 2012 15:11:35 -0800, Andrew Morton <> wrote:
> On Sun, 26 Feb 2012 23:49:58 +0530
> "Aneesh Kumar K.V" <> wrote:
> > From: "Aneesh Kumar K.V" <>
> >
> > Drop using inode->i_mutex from read, since that can result in deadlock with
> > mmap. Ideally we can extend the patch to make sure we don't increase i_size
> > in mmap. But that will break userspace, because application will have to now
> > use truncate(2) to increase i_size in hugetlbfs.
> >
> > AFAIU i_mutex was added in hugetlbfs_read as per
> >
> This patch comes somewhat out of the blue and I'm unsure what's going on.
> You say there's some (potential?) deadlock with mmap, but it is
> undescribed. Have people observed this deadlock? Has it caused
> lockdep warnings? Please update the changelog to fully describe the
> bug.

Viro explained the deadlock in detail here:

I will also update the commit message with this information.

> Also, the new truncate_sem is undoumented. This leaves readers to work
> out for themselves what it might be for. Please let's add code
> comments which completely describe the race, and how this lock prevents
> it.
> We should also document our locking rules. When should code take this
> lock? What are its ranking rules with respect to i_mutex, i_mmap_mutex
> and possibly others?

Will update the patch with these details


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