This function, in addition to be called by a generated_co_wrapper,
is also called by the blk_* API.
The strategy is to always take the lock at the function called
when the coroutine is created, to avoid recursive locking.

Protecting bdrv_co_pdiscard{_snapshot}() implies that the following BlockDriver
callbacks always called with graph rdlock taken:
- bdrv_co_pdiscard
- bdrv_aio_pdiscard
- bdrv_co_pdiscard_snapshot

Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
---
 block/block-backend.c            | 1 +
 block/io.c                       | 2 ++
 include/block/block_int-common.h | 3 +++
 3 files changed, 6 insertions(+)

diff --git a/block/block-backend.c b/block/block-backend.c
index d660772375..211a813523 100644
--- a/block/block-backend.c
+++ b/block/block-backend.c
@@ -1716,6 +1716,7 @@ blk_co_do_pdiscard(BlockBackend *blk, int64_t offset, int64_t bytes)
     IO_CODE();
 
     blk_wait_while_drained(blk);
+    GRAPH_RDLOCK_GUARD();
 
     ret = blk_check_byte_request(blk, offset, bytes);
     if (ret < 0) {
diff --git a/block/io.c b/block/io.c
index c9b451fecd..bc9f47538c 100644
--- a/block/io.c
+++ b/block/io.c
@@ -2885,6 +2885,7 @@ int coroutine_fn bdrv_co_pdiscard(BdrvChild *child, int64_t offset,
     int head, tail, align;
     BlockDriverState *bs = child->bs;
     IO_CODE();
+    assert_bdrv_graph_readable();
 
     if (!bs || !bs->drv || !bdrv_is_inserted(bs)) {
         return -ENOMEDIUM;
@@ -3488,6 +3489,7 @@ bdrv_co_pdiscard_snapshot(BlockDriverState *bs, int64_t offset, int64_t bytes)
     BlockDriver *drv = bs->drv;
     int ret;
     IO_CODE();
+    assert_bdrv_graph_readable();
 
     if (!drv) {
         return -ENOMEDIUM;
diff --git a/include/block/block_int-common.h b/include/block/block_int-common.h
index 568c2d3092..7c34a8e40f 100644
--- a/include/block/block_int-common.h
+++ b/include/block/block_int-common.h
@@ -486,6 +486,7 @@ struct BlockDriver {
         BdrvRequestFlags flags, BlockCompletionFunc *cb, void *opaque);
     BlockAIOCB *(*bdrv_aio_flush)(BlockDriverState *bs,
         BlockCompletionFunc *cb, void *opaque);
+    /* Called with graph rdlock taken. */
     BlockAIOCB *(*bdrv_aio_pdiscard)(BlockDriverState *bs,
         int64_t offset, int bytes,
         BlockCompletionFunc *cb, void *opaque);
@@ -559,6 +560,7 @@ struct BlockDriver {
      */
     int coroutine_fn (*bdrv_co_pwrite_zeroes)(BlockDriverState *bs,
         int64_t offset, int64_t bytes, BdrvRequestFlags flags);
+    /* Called with graph rdlock taken. */
     int coroutine_fn (*bdrv_co_pdiscard)(BlockDriverState *bs,
         int64_t offset, int64_t bytes);
 
@@ -647,6 +649,7 @@ struct BlockDriver {
     int coroutine_fn (*bdrv_co_snapshot_block_status)(BlockDriverState *bs,
         bool want_zero, int64_t offset, int64_t bytes, int64_t *pnum,
         int64_t *map, BlockDriverState **file);
+    /* Called with graph rdlock taken. */
     int coroutine_fn (*bdrv_co_pdiscard_snapshot)(BlockDriverState *bs,
         int64_t offset, int64_t bytes);
 
-- 
2.31.1