linux-kernel - Re: [PATCH v7 9/9] block: Avoid deadlocks with bio allocation by stacking drivers

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Message-ID: <20120829013150.GA9269@redhat.com>
Date:	Tue, 28 Aug 2012 21:31:50 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Kent Overstreet <koverstreet@...gle.com>
Cc:	Tejun Heo <tj@...nel.org>, linux-bcache@...r.kernel.org,
	linux-kernel@...r.kernel.org, dm-devel@...hat.com,
	mpatocka@...hat.com, bharrosh@...asas.com,
	Jens Axboe <axboe@...nel.dk>
Subject: Re: [PATCH v7 9/9] block: Avoid deadlocks with bio allocation by
 stacking drivers

On Tue, Aug 28, 2012 at 04:01:08PM -0700, Kent Overstreet wrote:
> On Tue, Aug 28, 2012 at 03:28:00PM -0700, Kent Overstreet wrote:
> > On Tue, Aug 28, 2012 at 01:49:10PM -0700, Tejun Heo wrote:
> > > Overall, I *think* this is correct but need to think more about it to
> > > be sure.
> > 
> > Please do. As much time as I've spent staring at this kind of stuff,
> > I'm pretty sure I've got it correct but it still makes my head hurt to
> > work out all the various possible deadlocks.
> 
> Hilarious thought: We're punting bios to a rescuer thread that's
> specific to a certain bio_set, right? What if we happen to punt bios
> from a different bio_set? And then the rescuer goes to resubmit those
> bios, and in the process they happen to have dependencies on the
> original bio_set...

Are they not fully allocated bios and when you submit these to underlying
device, ideally we should not be sharing memory pool at different layers
of stack otherwise we will deadlock any way as stack depth increases. So
there should not be a dependency on original bio_set?

Or, am I missing something. May be an example will help.

Thanks
Vivek
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