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

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Date:	Wed, 19 Oct 2011 10:49:52 -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 01:45:56PM -0400, Vivek Goyal wrote:
> > 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.
> 
> rcu protected spinlocks. Interesting. :-)

:)

> IIUC, it still leaves the window open that two callers think they have
> the spinlock. One is holding the driver provided lock and other is holding
> the queue private lock.

On blk_cleanup_queue() completion, the two are disjoint, so that
shouldn't be a problem.  Block layer won't call into low level drivers
and low level drivers aren't supposed to call into block layer either.

Thanks.

-- 
tejun
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