[Qemu-devel] [PATCH v2 12/18] block: allow waiting only for overlapping writes

[Paolo Bonzini <pbonzini@redhat.com>]

To implement mismatching block size, we will reuse the request tracking
mechanism that is used for copy-on-read.  However, waiting for overlapping
reads is not needed to protect against "torn reads", so add a flag to
wait_for_overlapping_requests.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 block.c |   13 ++++++++++---
 1 files changed, 10 insertions(+), 3 deletions(-)

diff --git a/block.c b/block.c
index af41fb3..76e1c6a 100644
--- a/block.c
+++ b/block.c
@@ -1198,7 +1198,7 @@ static bool tracked_request_overlaps(BdrvTrackedRequest *req,
 }
 
 static void coroutine_fn wait_for_overlapping_requests(BlockDriverState *bs,
-        int64_t sector_num, int nb_sectors)
+        int64_t sector_num, int nb_sectors, bool writes_only)
 {
     BdrvTrackedRequest *req;
     int64_t cluster_sector_num;
@@ -1214,9 +1214,16 @@ static void coroutine_fn wait_for_overlapping_requests(BlockDriverState *bs,
     round_to_clusters(bs, sector_num, nb_sectors,
                       &cluster_sector_num, &cluster_nb_sectors);
 
+    if (writes_only && !(bs->open_flags & BDRV_O_RDWR)) {
+        return;
+    }
+
     do {
         retry = false;
         QLIST_FOREACH(req, &bs->tracked_requests, list) {
+            if (writes_only && !req->is_write) {
+                continue;
+            }
             if (tracked_request_overlaps(req, cluster_sector_num,
                                          cluster_nb_sectors)) {
                 /* Hitting this means there was a reentrant request, for
@@ -1591,7 +1598,7 @@ static int coroutine_fn bdrv_co_do_readv(BlockDriverState *bs,
     }
 
     if (bs->copy_on_read_in_flight) {
-        wait_for_overlapping_requests(bs, sector_num, nb_sectors);
+        wait_for_overlapping_requests(bs, sector_num, nb_sectors, false);
     }
 
     tracked_request_begin(&req, bs, sector_num, nb_sectors, false);
@@ -1665,7 +1672,7 @@ static int coroutine_fn bdrv_co_do_writev(BlockDriverState *bs,
     }
 
     if (bs->copy_on_read_in_flight) {
-        wait_for_overlapping_requests(bs, sector_num, nb_sectors);
+        wait_for_overlapping_requests(bs, sector_num, nb_sectors, false);
     }
 
     tracked_request_begin(&req, bs, sector_num, nb_sectors, true);
-- 
1.7.7.6