Re: [PATCH] backing dev unplugging

From: Jens Axboe
Date: Wed Mar 10 2004 - 16:08:18 EST

On Wed, Mar 10 2004, Andrew Morton wrote:
> Jens Axboe <axboe@xxxxxxx> wrote:
> >
> > Here's a first cut at killing global plugging of block devices to reduce
> > the nasty contention blk_plug_lock caused.
> Shouldn't we take read_lock(&md->map_lock) in dm_table_unplug_all()?

Ugh yes, we certainly should.

> If so, what are the lock ranking issues here? The queue lock is not
> held yet, so it seems pretty simple?

As far as I can tell, it's pretty straight forward. The unplug_fn() will
grab the queue lock for 'ordinary' devices, for dm on dm you'd nest the
maplock inside each other (which should be quite alright, as far as I
can tell, without pulling any nasty tricks).

Jens Axboe

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