test-bdrv-drain: bdrv_drain() works with cross-AioContext events

As long as nobody keeps the other I/O thread from working, there is no
reason why bdrv_drain() wouldn't work with cross-AioContext events. The
key is that the root request we're waiting for is in the AioContext
we're polling (which it always is for bdrv_drain()) so that aio_poll()
is woken up in the end.

Add a test case that shows that it works. Remove the comment in
bdrv_drain() that claims otherwise.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/io.c

@@ -370,10 +370,6 @@ void bdrv_unapply_subtree_drain(BdrvChild *child, BlockDriverState *old_parent)
  *
  * Note that unlike bdrv_drain_all(), the caller must hold the BlockDriverState
  * AioContext.
- *
- * Only this BlockDriverState's AioContext is run, so in-flight requests must
- * not depend on events in other AioContexts.  In that case, use
- * bdrv_drain_all() instead.
  */
 void coroutine_fn bdrv_co_drain(BlockDriverState *bs)
 {

tests/test-bdrv-drain.c

@@ -27,9 +27,13 @@
 #include "block/blockjob_int.h"
 #include "sysemu/block-backend.h"
 #include "qapi/error.h"
+#include "iothread.h"
+
+static QemuEvent done_event;
 
 typedef struct BDRVTestState {
     int drain_count;
+    AioContext *bh_indirection_ctx;
 } BDRVTestState;
 
 static void coroutine_fn bdrv_test_co_drain_begin(BlockDriverState *bs)
@@ -50,16 +54,29 @@ static void bdrv_test_close(BlockDriverState *bs)
     g_assert_cmpint(s->drain_count, >, 0);
 }
 
+static void co_reenter_bh(void *opaque)
+{
+    aio_co_wake(opaque);
+}
+
 static int coroutine_fn bdrv_test_co_preadv(BlockDriverState *bs,
                                             uint64_t offset, uint64_t bytes,
                                             QEMUIOVector *qiov, int flags)
 {
+    BDRVTestState *s = bs->opaque;
+
     /* We want this request to stay until the polling loop in drain waits for
      * it to complete. We need to sleep a while as bdrv_drain_invoke() comes
      * first and polls its result, too, but it shouldn't accidentally complete
      * this request yet. */
     qemu_co_sleep_ns(QEMU_CLOCK_REALTIME, 100000);
 
+    if (s->bh_indirection_ctx) {
+        aio_bh_schedule_oneshot(s->bh_indirection_ctx, co_reenter_bh,
+                                qemu_coroutine_self());
+        qemu_coroutine_yield();
+    }
+
     return 0;
 }
 
@@ -490,6 +507,164 @@ static void test_graph_change(void)
     blk_unref(blk_b);
 }
 
+struct test_iothread_data {
+    BlockDriverState *bs;
+    enum drain_type drain_type;
+    int *aio_ret;
+};
+
+static void test_iothread_drain_entry(void *opaque)
+{
+    struct test_iothread_data *data = opaque;
+
+    aio_context_acquire(bdrv_get_aio_context(data->bs));
+    do_drain_begin(data->drain_type, data->bs);
+    g_assert_cmpint(*data->aio_ret, ==, 0);
+    do_drain_end(data->drain_type, data->bs);
+    aio_context_release(bdrv_get_aio_context(data->bs));
+
+    qemu_event_set(&done_event);
+}
+
+static void test_iothread_aio_cb(void *opaque, int ret)
+{
+    int *aio_ret = opaque;
+    *aio_ret = ret;
+    qemu_event_set(&done_event);
+}
+
+/*
+ * Starts an AIO request on a BDS that runs in the AioContext of iothread 1.
+ * The request involves a BH on iothread 2 before it can complete.
+ *
+ * @drain_thread = 0 means that do_drain_begin/end are called from the main
+ * thread, @drain_thread = 1 means that they are called from iothread 1. Drain
+ * for this BDS cannot be called from iothread 2 because only the main thread
+ * may do cross-AioContext polling.
+ */
+static void test_iothread_common(enum drain_type drain_type, int drain_thread)
+{
+    BlockBackend *blk;
+    BlockDriverState *bs;
+    BDRVTestState *s;
+    BlockAIOCB *acb;
+    int aio_ret;
+    struct test_iothread_data data;
+
+    IOThread *a = iothread_new();
+    IOThread *b = iothread_new();
+    AioContext *ctx_a = iothread_get_aio_context(a);
+    AioContext *ctx_b = iothread_get_aio_context(b);
+
+    QEMUIOVector qiov;
+    struct iovec iov = {
+        .iov_base = NULL,
+        .iov_len = 0,
+    };
+    qemu_iovec_init_external(&qiov, &iov, 1);
+
+    /* bdrv_drain_all() may only be called from the main loop thread */
+    if (drain_type == BDRV_DRAIN_ALL && drain_thread != 0) {
+        goto out;
+    }
+
+    blk = blk_new(BLK_PERM_ALL, BLK_PERM_ALL);
+    bs = bdrv_new_open_driver(&bdrv_test, "test-node", BDRV_O_RDWR,
+                              &error_abort);
+    s = bs->opaque;
+    blk_insert_bs(blk, bs, &error_abort);
+
+    blk_set_aio_context(blk, ctx_a);
+    aio_context_acquire(ctx_a);
+
+    s->bh_indirection_ctx = ctx_b;
+
+    aio_ret = -EINPROGRESS;
+    if (drain_thread == 0) {
+        acb = blk_aio_preadv(blk, 0, &qiov, 0, test_iothread_aio_cb, &aio_ret);
+    } else {
+        acb = blk_aio_preadv(blk, 0, &qiov, 0, aio_ret_cb, &aio_ret);
+    }
+    g_assert(acb != NULL);
+    g_assert_cmpint(aio_ret, ==, -EINPROGRESS);
+
+    aio_context_release(ctx_a);
+
+    data = (struct test_iothread_data) {
+        .bs         = bs,
+        .drain_type = drain_type,
+        .aio_ret    = &aio_ret,
+    };
+
+    switch (drain_thread) {
+    case 0:
+        if (drain_type != BDRV_DRAIN_ALL) {
+            aio_context_acquire(ctx_a);
+        }
+
+        /* The request is running on the IOThread a. Draining its block device
+         * will make sure that it has completed as far as the BDS is concerned,
+         * but the drain in this thread can continue immediately after
+         * bdrv_dec_in_flight() and aio_ret might be assigned only slightly
+         * later. */
+        qemu_event_reset(&done_event);
+        do_drain_begin(drain_type, bs);
+        g_assert_cmpint(bs->in_flight, ==, 0);
+
+        if (drain_type != BDRV_DRAIN_ALL) {
+            aio_context_release(ctx_a);
+        }
+        qemu_event_wait(&done_event);
+        if (drain_type != BDRV_DRAIN_ALL) {
+            aio_context_acquire(ctx_a);
+        }
+
+        g_assert_cmpint(aio_ret, ==, 0);
+        do_drain_end(drain_type, bs);
+
+        if (drain_type != BDRV_DRAIN_ALL) {
+            aio_context_release(ctx_a);
+        }
+        break;
+    case 1:
+        qemu_event_reset(&done_event);
+        aio_bh_schedule_oneshot(ctx_a, test_iothread_drain_entry, &data);
+        qemu_event_wait(&done_event);
+        break;
+    default:
+        g_assert_not_reached();
+    }
+
+    aio_context_acquire(ctx_a);
+    blk_set_aio_context(blk, qemu_get_aio_context());
+    aio_context_release(ctx_a);
+
+    bdrv_unref(bs);
+    blk_unref(blk);
+
+out:
+    iothread_join(a);
+    iothread_join(b);
+}
+
+static void test_iothread_drain_all(void)
+{
+    test_iothread_common(BDRV_DRAIN_ALL, 0);
+    test_iothread_common(BDRV_DRAIN_ALL, 1);
+}
+
+static void test_iothread_drain(void)
+{
+    test_iothread_common(BDRV_DRAIN, 0);
+    test_iothread_common(BDRV_DRAIN, 1);
+}
+
+static void test_iothread_drain_subtree(void)
+{
+    test_iothread_common(BDRV_SUBTREE_DRAIN, 0);
+    test_iothread_common(BDRV_SUBTREE_DRAIN, 1);
+}
+
 
 typedef struct TestBlockJob {
     BlockJob common;
@@ -618,10 +793,13 @@ static void test_blockjob_drain_subtree(void)
 
 int main(int argc, char **argv)
 {
+    int ret;
+
     bdrv_init();
     qemu_init_main_loop(&error_abort);
 
     g_test_init(&argc, &argv, NULL);
+    qemu_event_init(&done_event, false);
 
     g_test_add_func("/bdrv-drain/driver-cb/drain_all", test_drv_cb_drain_all);
     g_test_add_func("/bdrv-drain/driver-cb/drain", test_drv_cb_drain);
@@ -648,10 +826,17 @@ int main(int argc, char **argv)
     g_test_add_func("/bdrv-drain/multiparent", test_multiparent);
     g_test_add_func("/bdrv-drain/graph-change", test_graph_change);
 
+    g_test_add_func("/bdrv-drain/iothread/drain_all", test_iothread_drain_all);
+    g_test_add_func("/bdrv-drain/iothread/drain", test_iothread_drain);
+    g_test_add_func("/bdrv-drain/iothread/drain_subtree",
+                    test_iothread_drain_subtree);
+
     g_test_add_func("/bdrv-drain/blockjob/drain_all", test_blockjob_drain_all);
     g_test_add_func("/bdrv-drain/blockjob/drain", test_blockjob_drain);
     g_test_add_func("/bdrv-drain/blockjob/drain_subtree",
                     test_blockjob_drain_subtree);
 
-    return g_test_run();
+    ret = g_test_run();
+    qemu_event_destroy(&done_event);
+    return ret;
 }