Re: [PATCH, RFC] xip: use i_mutex for xip_file_fault

[Al Viro <viro@ZenIV.linux.org.uk>]

On Sun, Sep 11, 2011 at 12:15:04PM +0100, Al Viro wrote:

> write() grabs ->i_mutex on the file it's going to write to.  It uses
> copy_from_user() while holding ->i_mutex; that can end up calling ->fault().
> If your data comes from the same file mmapped in your address space, you
> have xip_write_fault() called while you are in xip_file_write() and *already*
> are holding ->i_mutex on the same inode.  With your patch it will, AFAICS,
> cheerfully deadlock.

Oh, wait...  You are only doing that to write side of pagefault?  That's
better, but not much:

thread 1: mmap the file, modify mapping
thread 2: write() to file

The former will do xip_write_fault() while holding ->mmap_sem.
The latter will do copy_from_user() from xip_file_write(), getting
pagefaults while holding ->i_mutex.

Note that we are grabbing ->mmap_sem and ->i_mutex in opposite orders.
I.e. that will deadlock on you - all you need is threads sharing the
address space.