[Qemu-devel] [RFC 4/6] block: request overlap detection

[Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>]

Detect overlapping requests and remember to align to cluster boundaries
if the image format uses them.  This assumes that allocating I/O is
performed in cluster granularity - which is true for qcow2, qed, etc.

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
---
 block.c |   39 +++++++++++++++++++++++++++++++++++++--
 1 files changed, 37 insertions(+), 2 deletions(-)

diff --git a/block.c b/block.c
index cc3202c..0c22741 100644
--- a/block.c
+++ b/block.c
@@ -1052,21 +1052,56 @@ static BdrvTrackedRequest *tracked_request_add(BlockDriverState *bs,
     return req;
 }
 
+/**
+ * Round a region to cluster boundaries
+ */
+static void round_to_clusters(BlockDriverState *bs,
+                              int64_t sector_num, int nb_sectors,
+                              int64_t *cluster_sector_num,
+                              int *cluster_nb_sectors)
+{
+    BlockDriverInfo bdi;
+
+    if (bdrv_get_info(bs, &bdi) < 0 || bdi.cluster_size == 0) {
+        *cluster_sector_num = sector_num;
+        *cluster_nb_sectors = nb_sectors;
+    } else {
+        int64_t c = bdi.cluster_size / BDRV_SECTOR_SIZE;
+        *cluster_sector_num = (sector_num / c) * c;
+        *cluster_nb_sectors = ((sector_num % c) + nb_sectors + c - 1) / c * c;
+    }
+}
+
 static bool tracked_request_overlaps(BdrvTrackedRequest *req,
                                      int64_t sector_num, int nb_sectors) {
-    return false; /* not yet implemented */
+    /*        aaaa   bbbb */
+    if (sector_num >= req->sector_num + req->nb_sectors) {
+        return false;
+    }
+    /* bbbb   aaaa        */
+    if (req->sector_num >= sector_num + nb_sectors) {
+        return false;
+    }
+    return true;
 }
 
 static void coroutine_fn wait_for_overlapping_requests(BlockDriverState *bs,
         int64_t sector_num, int nb_sectors)
 {
     BdrvTrackedRequest *req;
+    int64_t cluster_sector_num;
+    int cluster_nb_sectors;
     bool retry;
 
+    /* If we touch the same cluster it counts as an overlap */
+    round_to_clusters(bs, sector_num, nb_sectors,
+                      &cluster_sector_num, &cluster_nb_sectors);
+
     do {
         retry = false;
         QLIST_FOREACH(req, &bs->tracked_requests, list) {
-            if (tracked_request_overlaps(req, sector_num, nb_sectors)) {
+            if (tracked_request_overlaps(req, cluster_sector_num,
+                                         cluster_nb_sectors)) {
                 qemu_co_queue_wait(&req->wait_queue);
                 retry = true;
                 break;
-- 
1.7.6.3