[Qemu-devel] [PATCH 05/16] iothread: release AioContext 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.

Previously applied as a0710f7995f914e3044e5899bd8ff6c43c62f916 and
then reverted.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 async.c                     | 22 ++++------------------
 docs/multiple-iothreads.txt | 25 +++++++++++++++----------
 include/block/aio.h         |  3 ---
 iothread.c                  | 11 ++---------
 tests/test-aio.c            | 19 +++++++++++--------
 5 files changed, 32 insertions(+), 48 deletions(-)

diff --git a/async.c b/async.c
index 29f2845..e4ef8b8 100644
--- a/async.c
+++ b/async.c
@@ -84,8 +84,8 @@ int aio_bh_poll(AioContext *ctx)
          * aio_notify again if necessary.
          */
         if (!bh->deleted && atomic_xchg(&bh->scheduled, 0)) {
-            /* Idle BHs and the notify BH don't count as progress */
-            if (!bh->idle && bh != ctx->notify_dummy_bh) {
+            /* Idle BHs don't count as progress */
+            if (!bh->idle) {
                 ret = 1;
             }
             bh->idle = 0;
@@ -237,7 +237,6 @@ aio_ctx_finalize(GSource     *source)
 {
     AioContext *ctx = (AioContext *) source;
 
-    qemu_bh_delete(ctx->notify_dummy_bh);
     thread_pool_free(ctx->thread_pool);
 
     qemu_mutex_lock(&ctx->bh_lock);
@@ -522,19 +521,6 @@ static void aio_timerlist_notify(void *opaque)
     aio_notify(opaque);
 }
 
-static void aio_rfifolock_cb(void *opaque)
-{
-    AioContext *ctx = opaque;
-
-    /* Kick owner thread in case they are blocked in aio_poll() */
-    qemu_bh_schedule(ctx->notify_dummy_bh);
-}
-
-static void notify_dummy_bh(void *opaque)
-{
-    /* Do nothing, we were invoked just to force the event loop to iterate */
-}
-
 static void event_notifier_dummy_cb(EventNotifier *e)
 {
 }
@@ -563,10 +549,10 @@ AioContext *aio_context_new(Error **errp)
                            event_notifier_dummy_cb);
     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);
 
-    ctx->notify_dummy_bh = aio_bh_new(ctx, notify_dummy_bh, NULL);
+    qemu_event_init(&ctx->sync_io_event, true);
 
     return ctx;
 fail:
diff --git a/docs/multiple-iothreads.txt b/docs/multiple-iothreads.txt
index ac3e74c..22a89b2 100644
--- a/docs/multiple-iothreads.txt
+++ b/docs/multiple-iothreads.txt
@@ -103,13 +103,10 @@ a BH in the target AioContext beforehand and then call qemu_bh_schedule().  No
 acquire/release or locking is needed for the qemu_bh_schedule() call.  But be
 sure to acquire the AioContext for aio_bh_new() if necessary.
 
-The relationship between AioContext and the block layer
--------------------------------------------------------
-The AioContext originates from the QEMU block layer because it provides a
-scoped way of running event loop iterations until all work is done.  This
-feature is used to complete all in-flight block I/O requests (see
-bdrv_drain_all()).  Nowadays AioContext is a generic event loop that can be
-used by any QEMU subsystem.
+AioContext and the block layer
+------------------------------
+The AioContext originates from the QEMU block layer, even though nowadays
+AioContext is a generic event loop that can be used by any QEMU subsystem.
 
 The block layer has support for AioContext integrated.  Each BlockDriverState
 is associated with an AioContext using bdrv_set_aio_context() and
@@ -120,9 +117,17 @@ Block layer code must therefore expect to run in an IOThread and avoid using
 old APIs that implicitly use the main loop.  See the "How to program for
 IOThreads" above for information on how to do that.
 
-If main loop code such as a QMP function wishes to access a BlockDriverState it
-must first call aio_context_acquire(bdrv_get_aio_context(bs)) to ensure the
-IOThread does not run in parallel.
+If main loop code such as a QMP function wishes to access a BlockDriverState
+it must first call aio_context_acquire(bdrv_get_aio_context(bs)) to ensure
+that callbacks in the IOThread do not run in parallel.
+
+Code running in the monitor typically uses bdrv_drain() to ensure that
+past requests from the guest are completed.  When a block device is
+running in an IOThread, the IOThread can also process requests from
+the guest (via ioeventfd).  These requests have to be blocked with
+aio_disable_clients() before calling bdrv_drain().  You can then reenable
+guest requests with aio_enable_clients() before finally releasing the
+AioContext and completing the monitor command.
 
 Long-running jobs (usually in the form of coroutines) are best scheduled in the
 BlockDriverState's AioContext to avoid the need to acquire/release around each
diff --git a/include/block/aio.h b/include/block/aio.h
index 191bd3e..7223daf 100644
--- a/include/block/aio.h
+++ b/include/block/aio.h
@@ -114,9 +114,6 @@ struct AioContext {
     bool notified;
     EventNotifier notifier;
 
-    /* Scheduling this BH forces the event loop it iterate */
-    QEMUBH *notify_dummy_bh;
-
     /* Thread pool for performing work and receiving completion callbacks */
     struct ThreadPool *thread_pool;
 
diff --git a/iothread.c b/iothread.c
index 1150912..14a12cd 100644
--- a/iothread.c
+++ b/iothread.c
@@ -38,7 +38,6 @@ bool aio_context_in_iothread(AioContext *ctx)
 static void *iothread_run(void *opaque)
 {
     IOThread *iothread = opaque;
-    bool blocking;
 
     rcu_register_thread();
 
@@ -48,14 +47,8 @@ static void *iothread_run(void *opaque)
     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);
     }
 
     rcu_unregister_thread();
diff --git a/tests/test-aio.c b/tests/test-aio.c
index 6ccea98..41e30f7 100644
--- a/tests/test-aio.c
+++ b/tests/test-aio.c
@@ -99,6 +99,7 @@ static void event_ready_cb(EventNotifier *e)
 
 typedef struct {
     QemuMutex start_lock;
+    EventNotifier notifier;
     bool thread_acquired;
 } AcquireTestData;
 
@@ -110,6 +111,8 @@ 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);
 
@@ -124,20 +127,19 @@ static void set_event_notifier(AioContext *ctx, EventNotifier *notifier,
     aio_set_event_notifier(ctx, notifier, false, handler);
 }
 
-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);
-    set_event_notifier(ctx, &notifier, dummy_notifier_read);
+    event_notifier_init(&data.notifier, false);
+    set_event_notifier(ctx, &data.notifier, dummy_notifier_read);
     g_assert(!aio_poll(ctx, false)); /* consume aio_notify() */
 
     qemu_mutex_init(&data.start_lock);
@@ -151,12 +153,13 @@ 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);
-    set_event_notifier(ctx, &notifier, NULL);
-    event_notifier_cleanup(&notifier);
+    set_event_notifier(ctx, &data.notifier, NULL);
+    event_notifier_cleanup(&data.notifier);
 
     g_assert(data.thread_acquired);
 }
-- 
2.5.0