linux-kernel - Re: [PATCH 07/10] block: reorganize throtl_get_tg() and blk_throtl

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Message-ID: <20111019173053.GG25124@google.com>
Date:	Wed, 19 Oct 2011 10:30:53 -0700
From:	Tejun Heo <tj@...nel.org>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	axboe@...nel.dk, linux-kernel@...r.kernel.org, ctalbott@...gle.com
Subject: Re: [PATCH 07/10] block: reorganize throtl_get_tg() and
 blk_throtl_bio()

Hello,

On Wed, Oct 19, 2011 at 01:19:17PM -0400, Vivek Goyal wrote:
> > Currently, the code has different opportunistic checks which can catch
> > most of those cases but unfortunatly I think it just makes the bugs
> > more obscure.
> > 
> > That said, we probably should be switching to internal lock once
> > clenaup is complete.
> 
> So even switching to internal lock is racy. Christoph suggeted to break down
> this sharing of queue lock and driver lock and suggested always use
> internal queue lock and modify drivers to use their own lock and manage it.
> It makes sense though it might be lot of work to fix drivers.

Hmmmm, yeah, right, switching itself would be racy.  Maybe not sharing
is the solution, I don't know.  There's a way to make the switching
safe tho.  Sth like the following.

	lock_queue(q, flags)
	{
		spinlock_t *lock;

		local_irq_save(flags);
		lock = rcu_dereference_sched(q->queue_lock);
		spin_lock(lock);
	}

and on cleanup, do synchronize_sched() after lock switching.  But yeah
it still stinks.

Thanks.

-- 
tejun
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