linux-kernel - Re: Help with implementing some form of barriers in 3.0 kernels.

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Date:	Wed, 14 Sep 2011 10:32:47 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	Jan Beulich <JBeulich@...e.com>
Cc:	Jeremy Fitzhardinge <jeremy@...p.org>, hch@...radead.org,
	Jan Kara <jack@...e.cz>, linux-kernel@...r.kernel.org
Subject: Re: Help with implementing some form of barriers in 3.0 kernels.

> > +	if (drain) {
> > +		struct request_queue *q = bdev_get_queue(preq.bdev);
> > +		unsigned long flags;
> > +

> > +		/* Emulate the original behavior of write barriers */
> > +		spin_lock_irqsave(q->queue_lock, flags);
> > +		elv_drain_elevator(q);
> > +		__blk_run_queue(q);
> > +		spin_unlock_irqrestore(q->queue_lock, flags);
> > +	}
I also had to add:

diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c
index eaf49d1..20fddbc 100644
--- a/drivers/block/xen-blkback/blkback.c
+++ b/drivers/block/xen-blkback/blkback.c
@@ -679,6 +679,10 @@ static int dispatch_rw_block_io(struct xen_blkif *blkif,
 		struct request_queue *q = bdev_get_queue(preq.bdev);
 		unsigned long flags;
 
+		if (!q->elevator) {
+			__end_block_io_op(pending_req, -EOPNOTSUPP);
+			return -EOPNOTSUPP;
+		}
 		/* Emulate the original behavior of write barriers */
 		spin_lock_irqsave(q->queue_lock, flags);
 		elv_drain_elevator(q);
diff --git a/drivers/block/xen-blkback/common.h b/drivers/block/xen-blkback/common.h
index 52d8893..722714a 100644
--- a/drivers/block/xen-blkback/common.h
+++ b/drivers/block/xen-blkback/common.h
@@ -157,6 +157,7 @@ struct xen_vbd {
 	/* Cached size parameter. */
 	sector_t		size;
 	bool			flush_support;
+	bool			barrier_support;
 };
 
 struct backend_info;
diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c
index da1e27a..7189ecd 100644
--- a/drivers/block/xen-blkback/xenbus.c
+++ b/drivers/block/xen-blkback/xenbus.c
@@ -384,6 +384,9 @@ static int xen_vbd_create(struct xen_blkif *blkif, blkif_vdev_t handle,
 	if (q && q->flush_flags)
 		vbd->flush_support = true;
 
+	if (q && q->elevator && q->elevator->ops)
+		vbd->barrier_support = true;
+
 	DPRINTK("Successful creation of handle=%04x (dom=%u)\n",
 		handle, blkif->domid);
 	return 0;
@@ -728,7 +731,7 @@ again:
 	if (err)
 		goto abort;
 
-	err = xen_blkbk_barrier(xbt, be, be->blkif->vbd.flush_support);
+	err = xen_blkbk_barrier(xbt, be, be->blkif->vbd.barrier_support);
 
 	err = xen_blkbk_discard(xbt, be);


Otherwise it would crash.. Thought I am not sure why the elevator is not
set (the guest is on a LVM LV).
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