aio-wait.h: introduce AIO_WAIT_WHILE_UNLOCKED

Same as AIO_WAIT_WHILE macro, but if we are in the Main loop
do not release and then acquire ctx_ 's aiocontext.

Once all Aiocontext locks go away, this macro will replace
AIO_WAIT_WHILE.

Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Message-Id: <20220926093214.506243-5-eesposit@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

include/block/aio-wait.h

@@ -59,10 +59,13 @@ typedef struct {
 extern AioWait global_aio_wait;
 
 /**
- * AIO_WAIT_WHILE:
+ * AIO_WAIT_WHILE_INTERNAL:
  * @ctx: the aio context, or NULL if multiple aio contexts (for which the
  *       caller does not hold a lock) are involved in the polling condition.
  * @cond: wait while this conditional expression is true
+ * @unlock: whether to unlock and then lock again @ctx. This apples
+ * only when waiting for another AioContext from the main loop.
+ * Otherwise it's ignored.
  *
  * Wait while a condition is true.  Use this to implement synchronous
  * operations that require event loop activity.
@@ -75,7 +78,7 @@ extern AioWait global_aio_wait;
  * wait on conditions between two IOThreads since that could lead to deadlock,
  * go via the main loop instead.
  */
-#define AIO_WAIT_WHILE(ctx, cond) ({                               \
+#define AIO_WAIT_WHILE_INTERNAL(ctx, cond, unlock) ({              \
     bool waited_ = false;                                          \
     AioWait *wait_ = &global_aio_wait;                             \
     AioContext *ctx_ = (ctx);                                      \
@@ -92,11 +95,11 @@ extern AioWait global_aio_wait;
         assert(qemu_get_current_aio_context() ==                   \
                qemu_get_aio_context());                            \
         while ((cond)) {                                           \
-            if (ctx_) {                                            \
+            if (unlock && ctx_) {                                  \
                 aio_context_release(ctx_);                         \
             }                                                      \
             aio_poll(qemu_get_aio_context(), true);                \
-            if (ctx_) {                                            \
+            if (unlock && ctx_) {                                  \
                 aio_context_acquire(ctx_);                         \
             }                                                      \
             waited_ = true;                                        \
@@ -105,6 +108,12 @@ extern AioWait global_aio_wait;
     qatomic_dec(&wait_->num_waiters);                              \
     waited_; })
 
+#define AIO_WAIT_WHILE(ctx, cond)                                  \
+    AIO_WAIT_WHILE_INTERNAL(ctx, cond, true)
+
+#define AIO_WAIT_WHILE_UNLOCKED(ctx, cond)                         \
+    AIO_WAIT_WHILE_INTERNAL(ctx, cond, false)
+
 /**
  * aio_wait_kick:
  * Wake up the main thread if it is waiting on AIO_WAIT_WHILE().  During