Re: next-20130117 - kernel BUG with aio

[Kent Overstreet <koverstreet@google.com>]

On Thu, Jan 24, 2013 at 01:27:59PM -0800, Andrew Morton wrote:
> Please also take a look at Jan's recent
> http://www.spinics.net/lists/linux-fsdevel/msg61738.html and have a
> think about how this plays with your patchset.

I can't think of any possible interactions - none of my aio stuff messes
with the way the fput() happens; the aio code does call fput() when the
kiocb is freed and my patches do touch _that_ code but none of the
behaviour there changes.

Might be worth documenting this though, I can't think of any reason it'd
be obvious looking at the aio code that the fput() has to happen after
aio_complete(). As with the bugs I just sent you patches for it's not
terribly clear who owns what in the kiocb when.

Reading those patches though - the main change is to call
inode_dio_done() before calling aio_complete(). All inode_dio_done()
does though is issue a wakeup - to whatever called inode_dio_wait().

That means whatever called inode_dio_wait() needs its own ref on the
inode, and from a cursory glance at the code it is _not_ at all clear to
me that's the case - if inode_dio_wait() is merely finishing things for
that specific IO that need to be done in process context, I can easily
imagine it not being the case.

Assuming whatever does call inode_dio_wait() does have its own ref,
there was only a real use after free when nothing was waiting on the
inode.

Similarly for the ext4 code to write unwritten extents - and having seen
and helped chase a bug in that code before, that code _definitely_ needs
auditing.