Re: [PATCH 07/10] block: reorganize throtl_get_tg() and blk_throtl_bio()

[Vivek Goyal <vgoyal@redhat.com>]

On Wed, Oct 19, 2011 at 10:30:53AM -0700, Tejun Heo wrote:
> 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.

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.

Thanks
Vivek