From mboxrd@z Thu Jan  1 00:00:00 1970
From: Vivek Goyal <vgoyal-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
Subject: Re: 3.6-rc5 cgroups blkio throttle + md regression
Date: Thu, 20 Sep 2012 15:57:59 -0400
Message-ID: <20120920195759.GK4681@redhat.com>
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To: Tejun Heo <tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
Cc: Joseph Glanville <joseph.glanville-2MxvZkOi9dvvnOemgxGiVw@public.gmane.org>, cgroups <cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>

On Thu, Sep 20, 2012 at 12:20:38PM -0700, Tejun Heo wrote:
> On Thu, Sep 20, 2012 at 03:17:16PM -0400, Vivek Goyal wrote:
> > I suspect we are looping in retry code because bio based queues never come
> > out of bypass mode.
> > 
> >                 /*
> >                  * If queue was bypassing, we should retry.  Do so after a
> >                  * short msleep().  It isn't strictly necessary but queue
> >                  * can be bypassing for some time and it's always nice to
> >                  * avoid busy looping.
> >                  */
> >                 if (ret == -EBUSY) {
> >                         msleep(10);
> >                         ret = restart_syscall();
> >                 }
> 
> Yeah, I incorrectly assumed that bio based drivers would call
> blk_init_allocated_queue().  I think we need to move the initial
> bypass_end call to blk_register_queue().  Like the following.  Not
> completely sure yet tho.

I can confirm that with this patch, hang issue is gone. I tried it on
a dm device.

Thanks
Vivek
> 
> diff --git a/block/blk-core.c b/block/blk-core.c
> index 4b4dbdf..cbb019a 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -714,9 +714,6 @@ blk_init_allocated_queue(struct request_queue *q, request_fn_proc *rfn,
>  		return NULL;
>  
>  	blk_queue_congestion_threshold(q);
> -
> -	/* all done, end the initial bypass */
> -	blk_queue_bypass_end(q);
>  	return q;
>  }
>  EXPORT_SYMBOL(blk_init_allocated_queue);
> diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
> index 9628b29..f53802e 100644
> --- a/block/blk-sysfs.c
> +++ b/block/blk-sysfs.c
> @@ -527,6 +527,8 @@ int blk_register_queue(struct gendisk *disk)
>  	if (WARN_ON(!q))
>  		return -ENXIO;
>  
> +	blk_queue_bypass_end(q);
> +
>  	ret = blk_trace_init_sysfs(dev);
>  	if (ret)
>  		return ret;