It's safer to expand in_flight request to start before enter to
coroutine in synchronous wrappers and end after BDRV_POLL_WHILE loop.
Note that qemu_coroutine_enter may only schedule the coroutine in some
circumstances.

bdrv_make_zero update includes refactoring: move the whole loop into
coroutine, which has additional benefit of not create/enter new
coroutine on each iteration.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
---
 block/io.c | 54 +++++++++++++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 51 insertions(+), 3 deletions(-)

diff --git a/block/io.c b/block/io.c
index 3bc0daec33..cd5374e6c7 100644
--- a/block/io.c
+++ b/block/io.c
@@ -2740,8 +2740,11 @@ int bdrv_is_allocated_above(BlockDriverState *top, BlockDriverState *base,
  * BDRV_REQ_FUA).
  *
  * Returns < 0 on error, 0 on success. For error codes see bdrv_write().
+ *
+ * To be called between exactly one pair of bdrv_inc/dec_in_flight()
  */
-int bdrv_make_zero(BdrvChild *child, BdrvRequestFlags flags)
+static int coroutine_fn
+bdrv_do_make_zero(BdrvChild *child, BdrvRequestFlags flags)
 {
     int ret;
     int64_t target_size, bytes, offset = 0;
@@ -2757,7 +2760,8 @@ int bdrv_make_zero(BdrvChild *child, BdrvRequestFlags flags)
         if (bytes <= 0) {
             return 0;
         }
-        ret = bdrv_block_status(bs, offset, bytes, &bytes, NULL, NULL);
+        ret = bdrv_co_block_status(bs, true, false,
+                                   offset, bytes, &bytes, NULL, NULL);
         if (ret < 0) {
             return ret;
         }
@@ -2765,7 +2769,7 @@ int bdrv_make_zero(BdrvChild *child, BdrvRequestFlags flags)
             offset += bytes;
             continue;
         }
-        ret = bdrv_pwrite_zeroes(child, offset, bytes, flags);
+        ret = bdrv_do_pwrite_zeroes(child, offset, bytes, flags);
         if (ret < 0) {
             return ret;
         }
@@ -2773,6 +2777,50 @@ int bdrv_make_zero(BdrvChild *child, BdrvRequestFlags flags)
     }
 }
 
+typedef struct BdrvDoMakeZeroData {
+    BdrvChild *child;
+    BdrvRequestFlags flags;
+    int ret;
+    bool done;
+} BdrvDoMakeZeroData;
+
+/* To be called between exactly one pair of bdrv_inc/dec_in_flight() */
+static void coroutine_fn bdrv_make_zero_co_entry(void *opaque)
+{
+    BdrvDoMakeZeroData *data = opaque;
+
+    data->ret = bdrv_do_make_zero(data->child, data->flags);
+    data->done = true;
+    aio_wait_kick();
+}
+
+int bdrv_make_zero(BdrvChild *child, BdrvRequestFlags flags)
+{
+    int ret;
+
+    bdrv_inc_in_flight(child->bs);
+
+    if (qemu_in_coroutine()) {
+        /* Fast-path if already in coroutine context */
+        ret = bdrv_do_make_zero(child, flags);
+    } else {
+        BdrvDoMakeZeroData data = {
+            .child = child,
+            .flags = flags,
+            .done = false,
+        };
+        Coroutine *co = qemu_coroutine_create(bdrv_make_zero_co_entry, &data);
+
+        bdrv_coroutine_enter(child->bs, co);
+        BDRV_POLL_WHILE(child->bs, !data.done);
+        ret = data.ret;
+    }
+
+    bdrv_dec_in_flight(child->bs);
+
+    return ret;
+}
+
 typedef struct BdrvVmstateCo {
     BlockDriverState    *bs;
     QEMUIOVector        *qiov;
-- 
2.21.0

On 4/27/20 9:39 AM, Vladimir Sementsov-Ogievskiy wrote:
> It's safer to expand in_flight request to start before enter to
> coroutine in synchronous wrappers and end after BDRV_POLL_WHILE loop.
> Note that qemu_coroutine_enter may only schedule the coroutine in some
> circumstances.

See my wording suggestions earlier in the series.

> 
> bdrv_make_zero update includes refactoring: move the whole loop into
> coroutine, which has additional benefit of not create/enter new
> coroutine on each iteration.
> 
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> ---
>   block/io.c | 54 +++++++++++++++++++++++++++++++++++++++++++++++++++---
>   1 file changed, 51 insertions(+), 3 deletions(-)
> 

> +int bdrv_make_zero(BdrvChild *child, BdrvRequestFlags flags)
> +{
> +    int ret;
> +
> +    bdrv_inc_in_flight(child->bs);
> +
> +    if (qemu_in_coroutine()) {
> +        /* Fast-path if already in coroutine context */
> +        ret = bdrv_do_make_zero(child, flags);
> +    } else {
> +        BdrvDoMakeZeroData data = {
> +            .child = child,
> +            .flags = flags,
> +            .done = false,

Another case where the line '.done = false,' is optional, thanks to C 
semantics, but does not hurt to leave it in.

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org