Re: [RFC PATCH 2/2] dm: only initialize full request_queue for request-based device

[Mike Snitzer <snitzer@redhat.com>]

On Wed, May 19 2010 at 10:39am -0400,
Mike Snitzer <snitzer@redhat.com> wrote:

> > On Wed, May 19, 2010 at 1:57 AM, Kiyoshi Ueda <k-ueda@ct.jp.nec.com> wrote:
> > > Hi Mike,
> > >
> > > dm_table_setup_md_queue() may allocate memory with blocking mode.
> > > Blocking allocation inside exclusive _hash_lock can cause deadlock;
> > > e.g. when it has to wait for other dm devices to resume to free some
> > > memory.

<snip>

> We discussed this and I understand the scope of the problem now.
> 
> Just reiterating what you covered when you first pointed this issue out:
> 
> It could be that a table load gets blocked (waiting on a memory
> allocation).  The table load can take as long as it needs.  But we can't
> have it block holding the exclusive _hash_lock while blocking.  Having
> _hash_lock prevents further DM ioctls.  The table load's allocation may
> be blocking waiting for writeback to a DM device that will be resumed by
> another thread.
> 
> Thanks again for pointing this out; I'll work to arrive at an
> alternative locking scheme.  Likely introduce a lock local to the
> multiple_device (effectively the 'queue_lock' I had before).  But

s/multiple_device/mapped_device/

Mike