Re: [PATCH v3 06/33] util/async: aio_co_schedule(): support reschedule in same ctx

[Paolo Bonzini]

On 12/05/21 09:15, Vladimir Sementsov-Ogievskiy wrote:
> > >
> >
> > I don't understand.  Why doesn't aio_co_enter go through the ctx != 
> > qemu_get_current_aio_context() branch and just do aio_co_schedule?  
> > That was at least the idea behind aio_co_wake and aio_co_enter.
>
> Because ctx is exactly qemu_get_current_aio_context(), as we are not in 
> iothread but in nbd connection thread. So, 
> qemu_get_current_aio_context() returns qemu_aio_context.

So the problem is that threads other than the main thread and
the I/O thread should not return qemu_aio_context.  The vCPU thread
may need to return it when running with BQL taken, though.

Something like this (untested):

diff --git a/include/block/aio.h b/include/block/aio.h
index 5f342267d5..10fcae1515 100644
--- a/include/block/aio.h
+++ b/include/block/aio.h
@@ -691,10 +691,13 @@ void aio_co_enter(AioContext *ctx, struct Coroutine *co);
  * Return the AioContext whose event loop runs in the current thread.
  *
  * If called from an IOThread this will be the IOThread's AioContext.  If
- * called from another thread it will be the main loop AioContext.
+ * called from the main thread or with the "big QEMU lock" taken it
+ * will be the main loop AioContext.
  */
 AioContext *qemu_get_current_aio_context(void);
 
+void qemu_set_current_aio_context(AioContext *ctx);
+
 /**
  * aio_context_setup:
  * @ctx: the aio context
diff --git a/iothread.c b/iothread.c
index 7f086387be..22b967e77c 100644
--- a/iothread.c
+++ b/iothread.c
@@ -39,11 +39,23 @@ DECLARE_CLASS_CHECKERS(IOThreadClass, IOTHREAD,
 #define IOTHREAD_POLL_MAX_NS_DEFAULT 0ULL
 #endif
 
-static __thread IOThread *my_iothread;
+static __thread AioContext *my_aiocontext;
+
+void qemu_set_current_aio_context(AioContext *ctx)
+{
+    assert(!my_aiocontext);
+    my_aiocontext = ctx;
+}
 
 AioContext *qemu_get_current_aio_context(void)
 {
-    return my_iothread ? my_iothread->ctx : qemu_get_aio_context();
+    if (my_aiocontext) {
+        return my_aiocontext;
+    }
+    if (qemu_mutex_iothread_locked()) {
+        return qemu_get_aio_context();
+    }
+    return NULL;
 }
 
 static void *iothread_run(void *opaque)
@@ -56,7 +68,7 @@ static void *iothread_run(void *opaque)
      * in this new thread uses glib.
      */
     g_main_context_push_thread_default(iothread->worker_context);
-    my_iothread = iothread;
+    qemu_set_current_aio_context(iothread->ctx);
     iothread->thread_id = qemu_get_thread_id();
     qemu_sem_post(&iothread->init_done_sem);
 
diff --git a/stubs/iothread.c b/stubs/iothread.c
index 8cc9e28c55..25ff398894 100644
--- a/stubs/iothread.c
+++ b/stubs/iothread.c
@@ -6,3 +6,7 @@ AioContext *qemu_get_current_aio_context(void)
 {
     return qemu_get_aio_context();
 }
+
+void qemu_set_current_aio_context(AioContext *ctx)
+{
+}
diff --git a/tests/unit/iothread.c b/tests/unit/iothread.c
index afde12b4ef..cab38b3da8 100644
--- a/tests/unit/iothread.c
+++ b/tests/unit/iothread.c
@@ -30,13 +30,26 @@ struct IOThread {
     bool stopping;
 };
 
-static __thread IOThread *my_iothread;
+static __thread AioContext *my_aiocontext;
+
+void qemu_set_current_aio_context(AioContext *ctx)
+{
+    assert(!my_aiocontext);
+    my_aiocontext = ctx;
+}
 
 AioContext *qemu_get_current_aio_context(void)
 {
-    return my_iothread ? my_iothread->ctx : qemu_get_aio_context();
+    if (my_aiocontext) {
+        return my_aiocontext;
+    }
+    if (qemu_mutex_iothread_locked()) {
+        return qemu_get_aio_context();
+    }
+    return NULL;
 }
 
+
 static void iothread_init_gcontext(IOThread *iothread)
 {
     GSource *source;
@@ -54,7 +67,7 @@ static void *iothread_run(void *opaque)
 
     rcu_register_thread();
 
-    my_iothread = iothread;
+    qemu_set_current_aio_context(iothread->ctx);
     qemu_mutex_lock(&iothread->init_done_lock);
     iothread->ctx = aio_context_new(&error_abort);
 
diff --git a/util/main-loop.c b/util/main-loop.c
index d9c55df6f5..4ae5b23e99 100644
--- a/util/main-loop.c
+++ b/util/main-loop.c
@@ -170,6 +170,7 @@ int qemu_init_main_loop(Error **errp)
     if (!qemu_aio_context) {
         return -EMFILE;
     }
+    qemu_set_current_aio_context(qemu_aio_context);
     qemu_notify_bh = qemu_bh_new(notify_event_cb, NULL);
     gpollfds = g_array_new(FALSE, FALSE, sizeof(GPollFD));
     src = aio_get_g_source(qemu_aio_context);

If it works for you, I can post it as a formal patch.

Paolo