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

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

On Sun, Sep 11, 2011 at 10:25:03AM +0200, Marco Stornelli wrote:
> Il 10/09/2011 17:56, Al Viro ha scritto:
> >On Sat, Sep 10, 2011 at 05:31:39PM +0200, Marco Stornelli wrote:
> >>From: Marco Stornelli<marco.stornelli@gmail.com>
> >>
> >>There aren't sufficient sync points for a fs for xip operations. In
> >>particular for the mmap case. It can be not sufficient to lock/unlock
> >>to do some operation inside get_xip_mem callback. For xip_file_read
> >>it's really easy to write a fs specific wrapper, xip_file_write hold
> >>i_mutex so no problem. With this patch we can avoid concurrent truncate
> >>operation and xip mmap.
> >
> >Umm...  I really don't like that; what's going to happen if you have a file
> >mmapped and do write() to that file from address in that mapping?
> >
> 
> Nothing strange. There is a serialization of the operations. Maybe I
> don't understand the point here.

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.