[Qemu-devel] [RFC 3/6] block: wait for overlapping requests

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

When copy-on-read is enabled it is necessary to wait for overlapping
requests before issuing new requests.  This prevents races between the
copy-on-read and a write request.

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
---
 block.c |   39 +++++++++++++++++++++++++++++++++++++++
 1 files changed, 39 insertions(+), 0 deletions(-)

diff --git a/block.c b/block.c
index e624ac3..cc3202c 100644
--- a/block.c
+++ b/block.c
@@ -1001,6 +1001,7 @@ struct BdrvTrackedRequest {
     int nb_sectors;
     bool is_write;
     QLIST_ENTRY(BdrvTrackedRequest) list;
+    CoQueue wait_queue; /* coroutines blocked on this request */
 };
 
 /**
@@ -1014,6 +1015,12 @@ static void tracked_request_remove(BdrvTrackedRequest *req)
 {
     if (req) {
         QLIST_REMOVE(req, list);
+
+        /* Wake up all coroutines blocked on this request */
+        while (qemu_co_queue_next(&req->wait_queue)) {
+            /* Do nothing */
+        }
+
         g_free(req);
     }
 }
@@ -1038,12 +1045,36 @@ static BdrvTrackedRequest *tracked_request_add(BlockDriverState *bs,
     req->sector_num = sector_num;
     req->nb_sectors = nb_sectors;
     req->is_write = is_write;
+    qemu_co_queue_init(&req->wait_queue);
 
     QLIST_INSERT_HEAD(&bs->tracked_requests, req, list);
 
     return req;
 }
 
+static bool tracked_request_overlaps(BdrvTrackedRequest *req,
+                                     int64_t sector_num, int nb_sectors) {
+    return false; /* not yet implemented */
+}
+
+static void coroutine_fn wait_for_overlapping_requests(BlockDriverState *bs,
+        int64_t sector_num, int nb_sectors)
+{
+    BdrvTrackedRequest *req;
+    bool retry;
+
+    do {
+        retry = false;
+        QLIST_FOREACH(req, &bs->tracked_requests, list) {
+            if (tracked_request_overlaps(req, sector_num, nb_sectors)) {
+                qemu_co_queue_wait(&req->wait_queue);
+                retry = true;
+                break;
+            }
+        }
+    } while (retry);
+}
+
 /**
  * Enable tracking of incoming requests
  *
@@ -1360,6 +1391,10 @@ static int coroutine_fn bdrv_co_do_readv(BlockDriverState *bs,
         return -EIO;
     }
 
+    if (bs->copy_on_read) {
+        wait_for_overlapping_requests(bs, sector_num, nb_sectors);
+    }
+
     req = tracked_request_add(bs, sector_num, nb_sectors, false);
     ret = drv->bdrv_co_readv(bs, sector_num, nb_sectors, qiov);
     tracked_request_remove(req);
@@ -1394,6 +1429,10 @@ static int coroutine_fn bdrv_co_do_writev(BlockDriverState *bs,
         return -EIO;
     }
 
+    if (bs->copy_on_read) {
+        wait_for_overlapping_requests(bs, sector_num, nb_sectors);
+    }
+
     req = tracked_request_add(bs, sector_num, nb_sectors, true);
 
     ret = drv->bdrv_co_writev(bs, sector_num, nb_sectors, qiov);
-- 
1.7.6.3