[PATCH]: xen/blkback: Add support for old BARRIER requests -'feature-barrier', was "Help with implementing some form of barriers in 3.0kernels."

[Konrad Rzeszutek Wilk]

On Wed, Sep 14, 2011 at 10:32:47AM -0400, Konrad Rzeszutek Wilk wrote:

Let me just post the one that I've succesfully tested. Not sure
about the need for the check for q->elvator in the dispatch_rw_block_io
so if you guys want me to remove it I can certainly do it.

commit a5a12f729697161522ecae0564915cf4145cb65c
Author: Jan Kara <jack@xxxxxxx>
Date:   Tue Sep 13 11:44:07 2011 +0100

    xen/blkback: Add support for old BARRIER requests - 'feature-barrier'
    
    Recent kernels do not support BARRIER operation but only FLUSH operation. But
    older xenblk frontends still use the BARRIER operation to achieve data
    integrity requirements. So add support for BARRIER operation into xenblk
    backend so that all guests do not corrupt their filesystem on host crash.
    
    Signed-off-by: Jan Kara <jack@xxxxxxx>
    Signed-off-by: Jan Beulich <JBeulich@xxxxxxxx>
    [v1: Added some extra functions, and other cases]
    [v2: Added check for q->elevator]
    [v3: Changed EXPORT_SYMBOL to EXPORT_SYMBOL_GPL]
    Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>

diff --git a/block/elevator.c b/block/elevator.c
index a3b64bc..4e6c5b6 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -616,6 +616,7 @@ void elv_drain_elevator(struct request_queue *q)
 		       q->elevator->elevator_type->elevator_name, q->nr_sorted);
 	}
 }
+EXPORT_SYMBOL_GPL(elv_drain_elevator);
 
 /*
  * Call with queue lock held, interrupts disabled
diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c
index 9713d5a..573744e 100644
--- a/drivers/block/xen-blkback/blkback.c
+++ b/drivers/block/xen-blkback/blkback.c
@@ -464,6 +464,11 @@ static void __end_block_io_op(struct pending_req *pending_req, int error)
 		pr_debug(DRV_PFX "flush diskcache op failed, not supported\n");
 		xen_blkbk_flush_diskcache(XBT_NIL, pending_req->blkif->be, 0);
 		pending_req->status = BLKIF_RSP_EOPNOTSUPP;
+	} else if ((pending_req->operation == BLKIF_OP_WRITE_BARRIER) &&
+		   (error == -EOPNOTSUPP)) {
+		pr_debug(DRV_PFX "write barrier op failed, not supported\n");
+		xen_blkbk_barrier(XBT_NIL, pending_req->blkif->be, 0);
+		pending_req->status = BLKIF_RSP_EOPNOTSUPP;
 	} else if (error) {
 		pr_debug(DRV_PFX "Buffer not up-to-date at end of operation,"
 			 " error=%d\n", error);
@@ -590,6 +595,7 @@ static int dispatch_rw_block_io(struct xen_blkif *blkif,
 	struct bio *biolist[BLKIF_MAX_SEGMENTS_PER_REQUEST];
 	int i, nbio = 0;
 	int operation;
+	bool drain = false;
 	struct blk_plug plug;
 
 	switch (req->operation) {
@@ -601,6 +607,8 @@ static int dispatch_rw_block_io(struct xen_blkif *blkif,
 		blkif->st_wr_req++;
 		operation = WRITE_ODIRECT;
 		break;
+	case BLKIF_OP_WRITE_BARRIER:
+		drain = true;
 	case BLKIF_OP_FLUSH_DISKCACHE:
 		blkif->st_f_req++;
 		operation = WRITE_FLUSH;
@@ -609,7 +617,6 @@ static int dispatch_rw_block_io(struct xen_blkif *blkif,
 		blkif->st_ds_req++;
 		operation = REQ_DISCARD;
 		break;
-	case BLKIF_OP_WRITE_BARRIER:
 	default:
 		operation = 0; /* make gcc happy */
 		goto fail_response;
@@ -668,6 +675,21 @@ static int dispatch_rw_block_io(struct xen_blkif *blkif,
 		}
 	}
 
+	if (drain) {
+		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);
+		__blk_run_queue(q);
+		spin_unlock_irqrestore(q->queue_lock, flags);
+	}
+
 	/*
 	 * If we have failed at this point, we need to undo the M2P override,
 	 * set gnttab_set_unmap_op on all of the grant references and perform
diff --git a/drivers/block/xen-blkback/common.h b/drivers/block/xen-blkback/common.h
index bfb532e..9f1990b 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;
@@ -228,6 +229,8 @@ int xen_blkif_schedule(void *arg);
 
 int xen_blkbk_flush_diskcache(struct xenbus_transaction xbt,
 			      struct backend_info *be, int state);
+int xen_blkbk_barrier(struct xenbus_transaction xbt,
+		      struct backend_info *be, int state);
 
 struct xenbus_device *xen_blkbk_xenbus(struct backend_info *be);
 
diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c
index 2b3aef0..ec0975b 100644
--- a/drivers/block/xen-blkback/xenbus.c
+++ b/drivers/block/xen-blkback/xenbus.c
@@ -376,6 +376,9 @@ 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)
+		vbd->barrier_support = true;
+
 	DPRINTK("Successful creation of handle=%04x (dom=%u)\n",
 		handle, blkif->domid);
 	return 0;
@@ -421,6 +424,20 @@ int xen_blkbk_flush_diskcache(struct xenbus_transaction xbt,
 	return err;
 }
 
+int xen_blkbk_barrier(struct xenbus_transaction xbt,
+		      struct backend_info *be, int state)
+{
+	struct xenbus_device *dev = be->dev;
+	int err;
+
+	err = xenbus_printf(xbt, dev->nodename, "feature-barrier",
+			    "%d", state);
+	if (err)
+		xenbus_dev_fatal(dev, err, "writing feature-barrier");
+
+	return err;
+}
+
 int xen_blkbk_discard(struct xenbus_transaction xbt, struct backend_info *be)
 {
 	struct xenbus_device *dev = be->dev;
@@ -706,6 +723,8 @@ again:
 	if (err)
 		goto abort;
 
+	err = xen_blkbk_barrier(xbt, be, be->blkif->vbd.barrier_support);
+
 	err = xen_blkbk_discard(xbt, be);
 
 	err = xenbus_printf(xbt, dev->nodename, "sectors", "%llu",
--
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