Re: [PATCH] block: Fix lock unbalance caused by lock disconnect

[Asias He]

On 05/28/2012 06:20 PM, Tejun Heo wrote:
> Hello, Asias.
>
> On Mon, May 28, 2012 at 10:15:18AM +0800, Asias He wrote:
> > > I don't think the patch description is correct.  The lock switcihng is
> > > inherently broken and the patch doesn't really fix the problem
> > > although it *might* make the problem less likely.  Trying to switch
> > > locks while there are other accessors of the lock is simply broken, it
> > > can never work without outer synchronization.
> >
> > Since the lock switching is broken, is it a good idea to force all
> > the drivers to use the block layer provided lock? i.e. Change the
> > API from
> > blk_init_queue(rfn, driver_lock) to blk_init_queue(rfn). Any reason
> > not to use the block layer provided one.
>
> I think hch tried to do that a while ago.  Dunno what happened to the
> patches.  IIRC, the whole external lock thing was about sharing a
> single lock across different request_queues.  Not sure whether it's
> actually beneficial enough or just a crazy broken optimization.

Do we have any existing use case of sharing a single lock across 
different request_queues? What's point of this sharing. Christoph?

If nobody has any objections I'd like to make the patches. Jens, any 
comments?

> > > Your patch might make
> > > the problem somewhat less likely simply because queue draining makes a
> > > lot of request_queue users go away.
> >
> > Who will use the request_queue after blk_cleanup_queue()?
>
> Anyone who still holds a ref might try to issue a new request on a
> dead queue.  ie. blkdev with filesystem mounted goes away and the FS
> issues a new read request after blk_cleanup_queue() finishes drainig.

OK. Thanks for explaining.


--
Asias
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/