Re: [PATCH] hugetlbfs: Add new rw_semaphore to fix truncate/read race

["Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>]

On Mon, 27 Feb 2012 15:11:35 -0800, Andrew Morton <akpm@linux-foundation.org> wrote:
> On Sun, 26 Feb 2012 23:49:58 +0530
> "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com> wrote:
> 
> > From: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
> > 
> > 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
> > http://lkml.indiana.edu/hypermail/linux/kernel/0707.2/3066.html
> 
> 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:

http://mid.gmane.org/20120217002726.GL23916@ZenIV.linux.org.uk

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

Thanks
-aneesh

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