[Qemu-devel] [PATCH v3 6/9] qcow2: qcow2_co_preadv: improve locking

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

Background: decryption will be done in threads, to take benefit of it,
we should move it out of the lock first.

But let's go further: it turns out, that for locking around switch
cases we have only two variants: when we just do memset(0) not
releasing the lock (it is useless) and when we actually can handle the
whole case out of the lock. So, refactor the whole thing to reduce
locked code region and make it clean.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
---
 block/qcow2.c | 48 ++++++++++++++++++++++++------------------------
 1 file changed, 24 insertions(+), 24 deletions(-)

diff --git a/block/qcow2.c b/block/qcow2.c
index 2865903b0e..e650528af7 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -1883,6 +1883,7 @@ static coroutine_fn int qcow2_co_preadv(BlockDriverState *bs, uint64_t offset,
 
         ret = qcow2_get_cluster_offset(bs, offset, &cur_bytes, &cluster_offset);
         if (ret < 0) {
+            qemu_co_mutex_unlock(&s->lock);
             goto fail;
         }
 
@@ -1891,39 +1892,38 @@ static coroutine_fn int qcow2_co_preadv(BlockDriverState *bs, uint64_t offset,
         qemu_iovec_reset(&hd_qiov);
         qemu_iovec_concat(&hd_qiov, qiov, bytes_done, cur_bytes);
 
+        if (ret == QCOW2_CLUSTER_ZERO_PLAIN ||
+            ret == QCOW2_CLUSTER_ZERO_ALLOC ||
+            (ret == QCOW2_CLUSTER_UNALLOCATED && !bs->backing))
+        {
+            /* No sense in releasing the lock */
+
+            qemu_iovec_memset(&hd_qiov, 0, 0, cur_bytes);
+
+            bytes -= cur_bytes;
+            offset += cur_bytes;
+            bytes_done += cur_bytes;
+            continue;
+        }
+
+        qemu_co_mutex_unlock(&s->lock);
+
         switch (ret) {
         case QCOW2_CLUSTER_UNALLOCATED:
-
-            if (bs->backing) {
-                BLKDBG_EVENT(bs->file, BLKDBG_READ_BACKING_AIO);
-                qemu_co_mutex_unlock(&s->lock);
-                ret = bdrv_co_preadv(bs->backing, offset, cur_bytes,
-                                     &hd_qiov, 0);
-                qemu_co_mutex_lock(&s->lock);
-                if (ret < 0) {
-                    goto fail;
-                }
-            } else {
-                /* Note: in this case, no need to wait */
-                qemu_iovec_memset(&hd_qiov, 0, 0, cur_bytes);
+            BLKDBG_EVENT(bs->file, BLKDBG_READ_BACKING_AIO);
+            ret = bdrv_co_preadv(bs->backing, offset, cur_bytes, &hd_qiov, 0);
+            if (ret < 0) {
+                goto fail;
             }
             break;
 
-        case QCOW2_CLUSTER_ZERO_PLAIN:
-        case QCOW2_CLUSTER_ZERO_ALLOC:
-            qemu_iovec_memset(&hd_qiov, 0, 0, cur_bytes);
-            break;
-
         case QCOW2_CLUSTER_COMPRESSED:
-            qemu_co_mutex_unlock(&s->lock);
             ret = qcow2_co_preadv_compressed(bs, cluster_offset,
                                              offset, cur_bytes,
                                              &hd_qiov);
-            qemu_co_mutex_lock(&s->lock);
             if (ret < 0) {
                 goto fail;
             }
-
             break;
 
         case QCOW2_CLUSTER_NORMAL:
@@ -1956,11 +1956,9 @@ static coroutine_fn int qcow2_co_preadv(BlockDriverState *bs, uint64_t offset,
             }
 
             BLKDBG_EVENT(bs->file, BLKDBG_READ_AIO);
-            qemu_co_mutex_unlock(&s->lock);
             ret = bdrv_co_preadv(bs->file,
                                  cluster_offset + offset_in_cluster,
                                  cur_bytes, &hd_qiov, 0);
-            qemu_co_mutex_lock(&s->lock);
             if (ret < 0) {
                 goto fail;
             }
@@ -1991,12 +1989,14 @@ static coroutine_fn int qcow2_co_preadv(BlockDriverState *bs, uint64_t offset,
         bytes -= cur_bytes;
         offset += cur_bytes;
         bytes_done += cur_bytes;
+
+        qemu_co_mutex_lock(&s->lock);
     }
     ret = 0;
 
-fail:
     qemu_co_mutex_unlock(&s->lock);
 
+fail:
     qemu_iovec_destroy(&hd_qiov);
     qemu_vfree(cluster_data);
 
-- 
2.18.0