[RFC PATCH 2/3] block: Allow bdrv_run_co() from different AioContext

[Kevin Wolf <kwolf@redhat.com>]

Coroutine functions that are entered through bdrv_run_co() are already
safe to call from synchronous code in a different AioContext because
bdrv_coroutine_enter() will schedule them in the context of the node.

However, the coroutine fastpath still requires that we're already in the
right AioContext when called in coroutine context.

In order to make the behaviour more consistent and to make life a bit
easier for callers, let's check the AioContext and automatically move
the current coroutine around if we're not in the right context yet.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
---
 block/io.c | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/block/io.c b/block/io.c
index c1badaadc9..7808e8bdc0 100644
--- a/block/io.c
+++ b/block/io.c
@@ -895,8 +895,21 @@ static int bdrv_run_co(BlockDriverState *bs, CoroutineEntry *entry,
                        void *opaque, int *ret)
 {
     if (qemu_in_coroutine()) {
-        /* Fast-path if already in coroutine context */
+        Coroutine *self = qemu_coroutine_self();
+        AioContext *bs_ctx = bdrv_get_aio_context(bs);
+        AioContext *co_ctx = qemu_coroutine_get_aio_context(self);
+
+        if (bs_ctx != co_ctx) {
+            /* Move to the iothread of the node */
+            aio_co_schedule(bs_ctx, self);
+            qemu_coroutine_yield();
+        }
         entry(opaque);
+        if (bs_ctx != co_ctx) {
+            /* Move back to the original AioContext */
+            aio_co_schedule(bs_ctx, self);
+            qemu_coroutine_yield();
+        }
     } else {
         Coroutine *co = qemu_coroutine_create(entry, opaque);
         *ret = NOT_DONE;
-- 
2.25.3