nbd/server: Fix drained_poll to wake coroutine in right AioContext

nbd_drained_poll() generally runs in the main thread, not whatever
iothread the NBD server coroutine is meant to run in, so it can't
directly reenter the coroutines to wake them up.

The code seems to have the right intention, it specifies the correct
AioContext when it calls qemu_aio_coroutine_enter(). However, this
functions doesn't schedule the coroutine to run in that AioContext, but
it assumes it is already called in the home thread of the AioContext.

To fix this, add a new thread-safe qio_channel_wake_read() that can be
called in the main thread to wake up the coroutine in its AioContext,
and use this in nbd_drained_poll().

Cc: qemu-stable@nongnu.org
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20230517152834.277483-3-kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

include/io/channel.h

@@ -757,6 +757,16 @@ void qio_channel_detach_aio_context(QIOChannel *ioc);
 void coroutine_fn qio_channel_yield(QIOChannel *ioc,
                                     GIOCondition condition);
 
+/**
+ * qio_channel_wake_read:
+ * @ioc: the channel object
+ *
+ * If qio_channel_yield() is currently waiting for the channel to become
+ * readable, interrupt it and reenter immediately. This function is safe to call
+ * from any thread.
+ */
+void qio_channel_wake_read(QIOChannel *ioc);
+
 /**
  * qio_channel_wait:
  * @ioc: the channel object

io/channel.c

@@ -19,6 +19,7 @@
  */
 
 #include "qemu/osdep.h"
+#include "block/aio-wait.h"
 #include "io/channel.h"
 #include "qapi/error.h"
 #include "qemu/main-loop.h"
@@ -514,7 +515,11 @@ int qio_channel_flush(QIOChannel *ioc,
 static void qio_channel_restart_read(void *opaque)
 {
     QIOChannel *ioc = opaque;
-    Coroutine *co = ioc->read_coroutine;
+    Coroutine *co = qatomic_xchg(&ioc->read_coroutine, NULL);
+
+    if (!co) {
+        return;
+    }
 
     /* Assert that aio_co_wake() reenters the coroutine directly */
     assert(qemu_get_current_aio_context() ==
@@ -525,7 +530,11 @@ static void qio_channel_restart_read(void *opaque)
 static void qio_channel_restart_write(void *opaque)
 {
     QIOChannel *ioc = opaque;
-    Coroutine *co = ioc->write_coroutine;
+    Coroutine *co = qatomic_xchg(&ioc->write_coroutine, NULL);
+
+    if (!co) {
+        return;
+    }
 
     /* Assert that aio_co_wake() reenters the coroutine directly */
     assert(qemu_get_current_aio_context() ==
@@ -568,7 +577,11 @@ void qio_channel_detach_aio_context(QIOChannel *ioc)
 void coroutine_fn qio_channel_yield(QIOChannel *ioc,
                                     GIOCondition condition)
 {
+    AioContext *ioc_ctx = ioc->ctx ?: qemu_get_aio_context();
+
     assert(qemu_in_coroutine());
+    assert(in_aio_context_home_thread(ioc_ctx));
+
     if (condition == G_IO_IN) {
         assert(!ioc->read_coroutine);
         ioc->read_coroutine = qemu_coroutine_self();
@@ -580,18 +593,26 @@ void coroutine_fn qio_channel_yield(QIOChannel *ioc,
     }
     qio_channel_set_aio_fd_handlers(ioc);
     qemu_coroutine_yield();
+    assert(in_aio_context_home_thread(ioc_ctx));
 
     /* Allow interrupting the operation by reentering the coroutine other than
      * through the aio_fd_handlers. */
-    if (condition == G_IO_IN && ioc->read_coroutine) {
+    if (condition == G_IO_IN) {
-        ioc->read_coroutine = NULL;
+        assert(ioc->read_coroutine == NULL);
         qio_channel_set_aio_fd_handlers(ioc);
-    } else if (condition == G_IO_OUT && ioc->write_coroutine) {
+    } else if (condition == G_IO_OUT) {
-        ioc->write_coroutine = NULL;
+        assert(ioc->write_coroutine == NULL);
         qio_channel_set_aio_fd_handlers(ioc);
     }
 }
 
+void qio_channel_wake_read(QIOChannel *ioc)
+{
+    Coroutine *co = qatomic_xchg(&ioc->read_coroutine, NULL);
+    if (co) {
+        aio_co_wake(co);
+    }
+}
 
 static gboolean qio_channel_wait_complete(QIOChannel *ioc,
                                           GIOCondition condition,

nbd/server.c

@@ -1599,8 +1599,7 @@ static bool nbd_drained_poll(void *opaque)
              * enter it here so we don't depend on the client to wake it up.
              */
             if (client->recv_coroutine != NULL && client->read_yielding) {
-                qemu_aio_coroutine_enter(exp->common.ctx,
+                qio_channel_wake_read(client->ioc);
-                                         client->recv_coroutine);
             }
 
             return true;