xen/blkback: Support 'feature-barrier' aka old-style BARRIER requests.

We emulate the barrier requests by draining the outstanding bio's
and then sending the WRITE_FLUSH command. To drain the I/Os
we use the refcnt that is used during disconnect to wait for all
the I/Os before disconnecting from the frontend. We latch on its
value and if it reaches either the threshold for disconnect or when
there are no more outstanding I/Os, then we have drained all I/Os.

Suggested-by: Christopher Hellwig <hch@infradead.org>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

3 files changed, 58 insertions, 2 deletions

diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c
index e0dab614049c..184b1335c8e9 100644
--- a/drivers/block/xen-blkback/blkback.c
+++ b/drivers/block/xen-blkback/blkback.c
@@ -452,6 +452,23 @@ static void xen_blk_discard(struct xen_blkif *blkif, struct blkif_request *req)
         make_response(blkif, req->id, req->operation, status);
 }
 
+static void xen_blk_drain_io(struct xen_blkif *blkif)
+{
+        atomic_set(&blkif->drain, 1);
+        do {
+                wait_for_completion_interruptible_timeout(
+                                &blkif->drain_complete, HZ);
+
+                if (!atomic_read(&blkif->drain))
+                        break;
+                /* The initial value is one, and one refcnt taken at the
+                 * start of the xen_blkif_schedule thread. */
+                if (atomic_read(&blkif->refcnt) <= 2)
+                        break;
+        } while (!kthread_should_stop());
+        atomic_set(&blkif->drain, 0);
+}
+
 /*
  * Completion callback on the bio's. Called as bh->b_end_io()
  */
@@ -464,6 +481,11 @@ 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);
@@ -481,6 +503,10 @@ static void __end_block_io_op(struct pending_req *pending_req, int error)
                               pending_req->operation, pending_req->status);
                 xen_blkif_put(pending_req->blkif);
                 free_req(pending_req);
+                if (atomic_read(&pending_req->blkif->refcnt) <= 2) {
+                        if (atomic_read(&pending_req->blkif->drain))
+                                complete(&pending_req->blkif->drain_complete);
+                }
         }
 }
 
@@ -574,7 +600,6 @@ do_block_io_op(struct xen_blkif *blkif)
 
         return more_to_do;
 }
-
 /*
  * Transmutation of the 'struct blkif_request' to a proper 'struct bio'
  * and call the 'submit_bio' to pass it to the underlying storage.
@@ -591,6 +616,7 @@ static int dispatch_rw_block_io(struct xen_blkif *blkif,
         int i, nbio = 0;
         int operation;
         struct blk_plug plug;
+        bool drain = false;
 
         switch (req->operation) {
         case BLKIF_OP_READ:
@@ -601,6 +627,8 @@ 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 +637,6 @@ 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 +695,12 @@ static int dispatch_rw_block_io(struct xen_blkif *blkif,
                 }
         }
 
+        /* Wait on all outstanding I/O's and once that has been completed
+         * issue the WRITE_FLUSH.
+         */
+        if (drain)
+                xen_blk_drain_io(pending_req->blkif);
+
         /*
          * 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 1b1bc4458685..e638457d9de4 100644
--- a/drivers/block/xen-blkback/common.h
+++ b/drivers/block/xen-blkback/common.h
@@ -181,6 +181,9 @@ struct xen_blkif {
         atomic_t                refcnt;
 
         wait_queue_head_t       wq;
+        /* for barrier (drain) requests */
+        struct completion       drain_complete;
+        atomic_t                drain;
         /* One thread per one blkif. */
         struct task_struct      *xenblkd;
         unsigned int            waiting_reqs;
@@ -229,6 +232,8 @@ 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);
 
 static inline void blkif_get_x86_32_req(struct blkif_request *dst,
diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c
index 1c44b3254134..a6d43030b107 100644
--- a/drivers/block/xen-blkback/xenbus.c
+++ b/drivers/block/xen-blkback/xenbus.c
@@ -114,6 +114,8 @@ static struct xen_blkif *xen_blkif_alloc(domid_t domid)
         spin_lock_init(&blkif->blk_ring_lock);
         atomic_set(&blkif->refcnt, 1);
         init_waitqueue_head(&blkif->wq);
+        init_completion(&blkif->drain_complete);
+        atomic_set(&blkif->drain, 0);
         blkif->st_print = jiffies;
         init_waitqueue_head(&blkif->waiting_to_free);
 
@@ -474,6 +476,19 @@ kfree:
 out:
         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;
+}
 
 /*
  * Entry point to this code when a new device is created.  Allocate the basic
@@ -708,6 +723,9 @@ again:
 
         err = xen_blkbk_discard(xbt, be);
 
+        /* If we can't advertise it is OK. */
+        err = xen_blkbk_barrier(xbt, be, be->blkif->vbd.flush_support);
+
         err = xenbus_printf(xbt, dev->nodename, "sectors", "%llu",
                             (unsigned long long)vbd_sz(&be->blkif->vbd));
         if (err) {