[PATCH v2 01/10] drains: create bh only when polling

[Emanuele Giuseppe Esposito <eesposit@redhat.com>]

We need to prevent coroutines from calling BDRV_POLL_WHILE, because
it can create deadlocks. This is done by firstly creating a bottom half
and then yielding. The bh is then scheduled in the main loop, performs
the drain and polling, and then resumes the coroutine.

The problem is that currently we create coroutine and bh regardless
on whether we eventually poll or not. There is no need to do so,
if no poll takes place.

Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
---
 block/io.c | 31 +++++++++++++++++++------------
 1 file changed, 19 insertions(+), 12 deletions(-)

diff --git a/block/io.c b/block/io.c
index efc011ce65..4a3e8d037d 100644
--- a/block/io.c
+++ b/block/io.c
@@ -447,7 +447,7 @@ static void bdrv_do_drained_begin(BlockDriverState *bs, bool recursive,
 {
     BdrvChild *child, *next;
 
-    if (qemu_in_coroutine()) {
+    if (poll && qemu_in_coroutine()) {
         bdrv_co_yield_to_drain(bs, true, recursive, parent, ignore_bds_parents,
                                poll, NULL);
         return;
@@ -513,12 +513,6 @@ static void bdrv_do_drained_end(BlockDriverState *bs, bool recursive,
     int old_quiesce_counter;
 
     assert(drained_end_counter != NULL);
-
-    if (qemu_in_coroutine()) {
-        bdrv_co_yield_to_drain(bs, false, recursive, parent, ignore_bds_parents,
-                               false, drained_end_counter);
-        return;
-    }
     assert(bs->quiesce_counter > 0);
 
     /* Re-enable things in child-to-parent order */
@@ -541,11 +535,24 @@ static void bdrv_do_drained_end(BlockDriverState *bs, bool recursive,
     }
 }
 
+static void bdrv_do_drained_end_co(BlockDriverState *bs, bool recursive,
+                                   BdrvChild *parent, bool ignore_bds_parents,
+                                   int *drained_end_counter)
+{
+    if (qemu_in_coroutine()) {
+        bdrv_co_yield_to_drain(bs, false, recursive, parent, ignore_bds_parents,
+                               false, drained_end_counter);
+        return;
+    }
+
+    bdrv_do_drained_end(bs, recursive, parent, ignore_bds_parents, drained_end_counter);
+}
+
 void bdrv_drained_end(BlockDriverState *bs)
 {
     int drained_end_counter = 0;
     IO_OR_GS_CODE();
-    bdrv_do_drained_end(bs, false, NULL, false, &drained_end_counter);
+    bdrv_do_drained_end_co(bs, false, NULL, false, &drained_end_counter);
     BDRV_POLL_WHILE(bs, qatomic_read(&drained_end_counter) > 0);
 }
 
@@ -559,7 +566,7 @@ void bdrv_subtree_drained_end(BlockDriverState *bs)
 {
     int drained_end_counter = 0;
     IO_OR_GS_CODE();
-    bdrv_do_drained_end(bs, true, NULL, false, &drained_end_counter);
+    bdrv_do_drained_end_co(bs, true, NULL, false, &drained_end_counter);
     BDRV_POLL_WHILE(bs, qatomic_read(&drained_end_counter) > 0);
 }
 
@@ -580,7 +587,7 @@ void bdrv_unapply_subtree_drain(BdrvChild *child, BlockDriverState *old_parent)
     IO_OR_GS_CODE();
 
     for (i = 0; i < old_parent->recursive_quiesce_counter; i++) {
-        bdrv_do_drained_end(child->bs, true, child, false,
+        bdrv_do_drained_end_co(child->bs, true, child, false,
                             &drained_end_counter);
     }
 
@@ -703,7 +710,7 @@ void bdrv_drain_all_end_quiesce(BlockDriverState *bs)
     g_assert(!bs->refcnt);
 
     while (bs->quiesce_counter) {
-        bdrv_do_drained_end(bs, false, NULL, true, &drained_end_counter);
+        bdrv_do_drained_end_co(bs, false, NULL, true, &drained_end_counter);
     }
     BDRV_POLL_WHILE(bs, qatomic_read(&drained_end_counter) > 0);
 }
@@ -727,7 +734,7 @@ void bdrv_drain_all_end(void)
         AioContext *aio_context = bdrv_get_aio_context(bs);
 
         aio_context_acquire(aio_context);
-        bdrv_do_drained_end(bs, false, NULL, true, &drained_end_counter);
+        bdrv_do_drained_end_co(bs, false, NULL, true, &drained_end_counter);
         aio_context_release(aio_context);
     }
 
-- 
2.31.1