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

From: Mike Snitzer
Date: Wed May 19 2010 - 10:46:02 EST

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

> > On Wed, May 19, 2010 at 1:57 AM, Kiyoshi Ueda <k-ueda@xxxxxxxxxxxxx> 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.


> 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


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