aio: warn about iohandler_ctx special casing

The main loop has two AioContexts: qemu_aio_context and iohandler_ctx.
The main loop runs them both, but nested aio_poll() calls on
qemu_aio_context exclude iohandler_ctx.

Which one should qemu_get_current_aio_context() return when called from
the main loop? Document that it's always qemu_aio_context.

This has subtle effects on functions that use
qemu_get_current_aio_context(). For example, aio_co_reschedule_self()
does not work when moving from iohandler_ctx to qemu_aio_context because
qemu_get_current_aio_context() does not differentiate these two
AioContexts.

Document this in order to reduce the chance of future bugs.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-ID: <20240506190622.56095-3-stefanha@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

include/block/aio.h

@@ -629,6 +629,9 @@ void aio_co_schedule(AioContext *ctx, Coroutine *co);
  *
  * Move the currently running coroutine to new_ctx. If the coroutine is already
  * running in new_ctx, do nothing.
+ *
+ * Note that this function cannot reschedule from iohandler_ctx to
+ * qemu_aio_context.
  */
 void coroutine_fn aio_co_reschedule_self(AioContext *new_ctx);
 
@@ -661,6 +664,9 @@ void aio_co_enter(AioContext *ctx, Coroutine *co);
  * If called from an IOThread this will be the IOThread's AioContext.  If
  * called from the main thread or with the "big QEMU lock" taken it
  * will be the main loop AioContext.
+ *
+ * Note that the return value is never the main loop's iohandler_ctx and the
+ * return value is the main loop AioContext instead.
  */
 AioContext *qemu_get_current_aio_context(void);