[Qemu-devel] [PATCH 3/3] iothread: release iothread around aio_poll

[Paolo Bonzini <pbonzini@redhat.com>]

This is the first step towards having fine-grained critical sections in
dataplane threads, which resolves lock ordering problems between
address_space_* functions (which need the BQL when doing MMIO, even
after we complete RCU-based dispatch) and the AioContext.

Because AioContext does not use contention callbacks anymore, the
unit test has to be changed.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 async.c          |  8 +-------
 iothread.c       | 11 ++---------
 tests/test-aio.c | 21 ++++++++++++---------
 3 files changed, 15 insertions(+), 25 deletions(-)

diff --git a/async.c b/async.c
index 0463dc4..e8a4c8b 100644
--- a/async.c
+++ b/async.c
@@ -289,12 +289,6 @@ 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;
@@ -312,7 +306,7 @@ 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, aio_rfifolock_cb, ctx);
+    rfifolock_init(&ctx->lock, NULL, NULL);
     timerlistgroup_init(&ctx->tlg, aio_timerlist_notify, ctx);
 
     return ctx;
diff --git a/iothread.c b/iothread.c
index 342a23f..a1f9109 100644
--- a/iothread.c
+++ b/iothread.c
@@ -31,21 +31,14 @@ 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 (!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);
+    while (!atomic_read(&iothread->stopping)) {
+        aio_poll(iothread->ctx, true);
     }
     return NULL;
 }
diff --git a/tests/test-aio.c b/tests/test-aio.c
index a7cb5c9..f88a042 100644
--- a/tests/test-aio.c
+++ b/tests/test-aio.c
@@ -107,6 +107,7 @@ static void test_notify(void)
 
 typedef struct {
     QemuMutex start_lock;
+    EventNotifier notifier;
     bool thread_acquired;
 } AcquireTestData;
 
@@ -118,6 +119,8 @@ 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);
 
@@ -126,20 +129,19 @@ static void *test_acquire_thread(void *opaque)
     return NULL;
 }
 
-static void dummy_notifier_read(EventNotifier *unused)
+static void dummy_notifier_read(EventNotifier *n)
 {
-    g_assert(false); /* should never be invoked */
+    event_notifier_test_and_clear(n);
 }
 
 static void test_acquire(void)
 {
     QemuThread thread;
-    EventNotifier notifier;
     AcquireTestData data;
 
     /* Dummy event notifier ensures aio_poll() will block */
-    event_notifier_init(&notifier, false);
-    aio_set_event_notifier(ctx, &notifier, dummy_notifier_read);
+    event_notifier_init(&data.notifier, false);
+    aio_set_event_notifier(ctx, &data.notifier, dummy_notifier_read);
     g_assert(!aio_poll(ctx, false)); /* consume aio_notify() */
 
     qemu_mutex_init(&data.start_lock);
@@ -150,12 +152,13 @@ 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(aio_poll(ctx, true));
+    g_assert(!data.thread_acquired);
     aio_context_release(ctx);
 
     qemu_thread_join(&thread);
-    aio_set_event_notifier(ctx, &notifier, NULL);
-    event_notifier_cleanup(&notifier);
+    aio_set_event_notifier(ctx, &data.notifier, NULL);
+    event_notifier_cleanup(&data.notifier);
 
     g_assert(data.thread_acquired);
 }
-- 
2.3.0