Revert "iothread: release iothread around aio_poll"

This reverts commit a0710f7995.

In qemu-devel email message <556DBF87.2020908@de.ibm.com>, Christian
Borntraeger writes:

  Having many guests all with a kernel/ramdisk (via -kernel) and
  several null block devices will result in hangs. All hanging
  guests are in partition detection code waiting for an I/O to return
  so very early maybe even the first I/O.

  Reverting that commit "fixes" the hangs.

Reverting this commit for the 2.4 release.  More time is needed to
investigate and correct this patch.

Reported-by: Christian Borntraeger <borntraeger@de.ibm.com>
Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

async.c

@@ -280,6 +280,12 @@ static void aio_timerlist_notify(void *opaque)
     aio_notify(opaque);
 }
 
+static void aio_rfifolock_cb(void *opaque)
+{
+    /* Kick owner thread in case they are blocked in aio_poll() */
+    aio_notify(opaque);
+}
+
 AioContext *aio_context_new(Error **errp)
 {
     int ret;
@@ -297,7 +303,7 @@ AioContext *aio_context_new(Error **errp)
                            event_notifier_test_and_clear);
     ctx->thread_pool = NULL;
     qemu_mutex_init(&ctx->bh_lock);
-    rfifolock_init(&ctx->lock, NULL, NULL);
+    rfifolock_init(&ctx->lock, aio_rfifolock_cb, ctx);
     timerlistgroup_init(&ctx->tlg, aio_timerlist_notify, ctx);
 
     return ctx;

iothread.c

@@ -31,14 +31,21 @@ typedef ObjectClass IOThreadClass;
 static void *iothread_run(void *opaque)
 {
     IOThread *iothread = opaque;
+    bool blocking;
 
     qemu_mutex_lock(&iothread->init_done_lock);
     iothread->thread_id = qemu_get_thread_id();
     qemu_cond_signal(&iothread->init_done_cond);
     qemu_mutex_unlock(&iothread->init_done_lock);
 
-    while (!atomic_read(&iothread->stopping)) {
-        aio_poll(iothread->ctx, true);
+    while (!iothread->stopping) {
+        aio_context_acquire(iothread->ctx);
+        blocking = true;
+        while (!iothread->stopping && aio_poll(iothread->ctx, blocking)) {
+            /* Progress was made, keep going */
+            blocking = false;
+        }
+        aio_context_release(iothread->ctx);
     }
     return NULL;
 }

tests/test-aio.c

@@ -107,7 +107,6 @@ static void test_notify(void)
 
 typedef struct {
     QemuMutex start_lock;
-    EventNotifier notifier;
     bool thread_acquired;
 } AcquireTestData;
 
@@ -119,8 +118,6 @@ static void *test_acquire_thread(void *opaque)
     qemu_mutex_lock(&data->start_lock);
     qemu_mutex_unlock(&data->start_lock);
 
-    g_usleep(500000);
-    event_notifier_set(&data->notifier);
     aio_context_acquire(ctx);
     aio_context_release(ctx);
 
@@ -129,19 +126,20 @@ static void *test_acquire_thread(void *opaque)
     return NULL;
 }
 
-static void dummy_notifier_read(EventNotifier *n)
+static void dummy_notifier_read(EventNotifier *unused)
 {
-    event_notifier_test_and_clear(n);
+    g_assert(false); /* should never be invoked */
 }
 
 static void test_acquire(void)
 {
     QemuThread thread;
+    EventNotifier notifier;
     AcquireTestData data;
 
     /* Dummy event notifier ensures aio_poll() will block */
-    event_notifier_init(&data.notifier, false);
-    aio_set_event_notifier(ctx, &data.notifier, dummy_notifier_read);
+    event_notifier_init(&notifier, false);
+    aio_set_event_notifier(ctx, &notifier, dummy_notifier_read);
     g_assert(!aio_poll(ctx, false)); /* consume aio_notify() */
 
     qemu_mutex_init(&data.start_lock);
@@ -155,13 +153,12 @@ static void test_acquire(void)
     /* Block in aio_poll(), let other thread kick us and acquire context */
     aio_context_acquire(ctx);
     qemu_mutex_unlock(&data.start_lock); /* let the thread run */
-    g_assert(aio_poll(ctx, true));
-    g_assert(!data.thread_acquired);
+    g_assert(!aio_poll(ctx, true));
     aio_context_release(ctx);
 
     qemu_thread_join(&thread);
-    aio_set_event_notifier(ctx, &data.notifier, NULL);
-    event_notifier_cleanup(&data.notifier);
+    aio_set_event_notifier(ctx, &notifier, NULL);
+    event_notifier_cleanup(&notifier);
 
     g_assert(data.thread_acquired);
 }