[PATCH 02/15] Revert "block: Don't poll in bdrv_replace_child_noperm()"

[Paolo Bonzini <pbonzini@redhat.com>]

This reverts commit a4e5c80a45b22359cf9c187f0df4f8544812c55c.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 block.c                      | 103 +++++------------------------------
 block/io.c                   |   2 +-
 include/block/block-io.h     |   8 ---
 tests/unit/test-bdrv-drain.c |  10 ----
 4 files changed, 15 insertions(+), 108 deletions(-)

diff --git a/block.c b/block.c
index 87022f4cd971..2f2123f4a4e5 100644
--- a/block.c
+++ b/block.c
@@ -2367,20 +2367,6 @@ static void bdrv_replace_child_abort(void *opaque)
 
     GLOBAL_STATE_CODE();
     /* old_bs reference is transparently moved from @s to @s->child */
-    if (!s->child->bs) {
-        /*
-         * The parents were undrained when removing old_bs from the child. New
-         * requests can't have been made, though, because the child was empty.
-         *
-         * TODO Make bdrv_replace_child_noperm() transactionable to avoid
-         * undraining the parent in the first place. Once this is done, having
-         * new_bs drained when calling bdrv_replace_child_tran() is not a
-         * requirement any more.
-         */
-        bdrv_parent_drained_begin_single(s->child, false);
-        assert(!bdrv_parent_drained_poll_single(s->child));
-    }
-    assert(s->child->quiesced_parent);
     bdrv_replace_child_noperm(s->child, s->old_bs);
     bdrv_unref(new_bs);
 }
@@ -2396,19 +2382,12 @@ static TransactionActionDrv bdrv_replace_child_drv = {
  *
  * Note: real unref of old_bs is done only on commit.
  *
- * Both @child->bs and @new_bs (if non-NULL) must be drained. @new_bs must be
- * kept drained until the transaction is completed.
- *
  * The function doesn't update permissions, caller is responsible for this.
  */
 static void bdrv_replace_child_tran(BdrvChild *child, BlockDriverState *new_bs,
                                     Transaction *tran)
 {
     BdrvReplaceChildState *s = g_new(BdrvReplaceChildState, 1);
-
-    assert(child->quiesced_parent);
-    assert(!new_bs || new_bs->quiesce_counter);
-
     *s = (BdrvReplaceChildState) {
         .child = child,
         .old_bs = child->bs,
@@ -2831,14 +2810,6 @@ uint64_t bdrv_qapi_perm_to_blk_perm(BlockPermission qapi_perm)
     return permissions[qapi_perm];
 }
 
-/*
- * Replaces the node that a BdrvChild points to without updating permissions.
- *
- * If @new_bs is non-NULL, the parent of @child must already be drained through
- * @child.
- *
- * This function does not poll.
- */
 static void bdrv_replace_child_noperm(BdrvChild *child,
                                       BlockDriverState *new_bs)
 {
@@ -2846,28 +2817,6 @@ static void bdrv_replace_child_noperm(BdrvChild *child,
     int new_bs_quiesce_counter;
 
     assert(!child->frozen);
-
-    /*
-     * If we want to change the BdrvChild to point to a drained node as its new
-     * child->bs, we need to make sure that its new parent is drained, too. In
-     * other words, either child->quiesce_parent must already be true or we must
-     * be able to set it and keep the parent's quiesce_counter consistent with
-     * that, but without polling or starting new requests (this function
-     * guarantees that it doesn't poll, and starting new requests would be
-     * against the invariants of drain sections).
-     *
-     * To keep things simple, we pick the first option (child->quiesce_parent
-     * must already be true). We also generalise the rule a bit to make it
-     * easier to verify in callers and more likely to be covered in test cases:
-     * The parent must be quiesced through this child even if new_bs isn't
-     * currently drained.
-     *
-     * The only exception is for callers that always pass new_bs == NULL. In
-     * this case, we obviously never need to consider the case of a drained
-     * new_bs, so we can keep the callers simpler by allowing them not to drain
-     * the parent.
-     */
-    assert(!new_bs || child->quiesced_parent);
     assert(old_bs != new_bs);
     GLOBAL_STATE_CODE();
 
@@ -2875,6 +2824,15 @@ static void bdrv_replace_child_noperm(BdrvChild *child,
         assert(bdrv_get_aio_context(old_bs) == bdrv_get_aio_context(new_bs));
     }
 
+    /*
+     * If the new child node is drained but the old one was not, flush
+     * all outstanding requests to the old child node.
+     */
+    new_bs_quiesce_counter = (new_bs ? new_bs->quiesce_counter : 0);
+    if (new_bs_quiesce_counter && !child->quiesced_parent) {
+        bdrv_parent_drained_begin_single(child, true);
+    }
+
     if (old_bs) {
         if (child->klass->detach) {
             child->klass->detach(child);
@@ -2894,9 +2852,11 @@ static void bdrv_replace_child_noperm(BdrvChild *child,
     }
 
     /*
-     * If the parent was drained through this BdrvChild previously, but new_bs
-     * is not drained, allow requests to come in only after the new node has
-     * been attached.
+     * If the old child node was drained but the new one is not, allow
+     * requests to come in only after the new node has been attached.
+     *
+     * Update new_bs_quiesce_counter because bdrv_parent_drained_begin_single()
+     * polls, which could have changed the value.
      */
     new_bs_quiesce_counter = (new_bs ? new_bs->quiesce_counter : 0);
     if (!new_bs_quiesce_counter && child->quiesced_parent) {
@@ -3033,24 +2993,6 @@ static BdrvChild *bdrv_attach_child_common(BlockDriverState *child_bs,
     }
 
     bdrv_ref(child_bs);
-    /*
-     * Let every new BdrvChild start with a drained parent. Inserting the child
-     * in the graph with bdrv_replace_child_noperm() will undrain it if
-     * @child_bs is not drained.
-     *
-     * The child was only just created and is not yet visible in global state
-     * until bdrv_replace_child_noperm() inserts it into the graph, so nobody
-     * could have sent requests and polling is not necessary.
-     *
-     * Note that this means that the parent isn't fully drained yet, we only
-     * stop new requests from coming in. This is fine, we don't care about the
-     * old requests here, they are not for this child. If another place enters a
-     * drain section for the same parent, but wants it to be fully quiesced, it
-     * will not run most of the the code in .drained_begin() again (which is not
-     * a problem, we already did this), but it will still poll until the parent
-     * is fully quiesced, so it will not be negatively affected either.
-     */
-    bdrv_parent_drained_begin_single(new_child, false);
     bdrv_replace_child_noperm(new_child, child_bs);
 
     BdrvAttachChildCommonState *s = g_new(BdrvAttachChildCommonState, 1);
@@ -5096,24 +5038,12 @@ static void bdrv_remove_child(BdrvChild *child, Transaction *tran)
     }
 
     if (child->bs) {
-        BlockDriverState *bs = child->bs;
-        bdrv_drained_begin(bs);
         bdrv_replace_child_tran(child, NULL, tran);
-        bdrv_drained_end(bs);
     }
 
     tran_add(tran, &bdrv_remove_child_drv, child);
 }
 
-static void undrain_on_clean_cb(void *opaque)
-{
-    bdrv_drained_end(opaque);
-}
-
-static TransactionActionDrv undrain_on_clean = {
-    .clean = undrain_on_clean_cb,
-};
-
 static int bdrv_replace_node_noperm(BlockDriverState *from,
                                     BlockDriverState *to,
                                     bool auto_skip, Transaction *tran,
@@ -5123,11 +5053,6 @@ static int bdrv_replace_node_noperm(BlockDriverState *from,
 
     GLOBAL_STATE_CODE();
 
-    bdrv_drained_begin(from);
-    bdrv_drained_begin(to);
-    tran_add(tran, &undrain_on_clean, from);
-    tran_add(tran, &undrain_on_clean, to);
-
     QLIST_FOREACH_SAFE(c, &from->parents, next_parent, next) {
         assert(c->bs == from);
         if (!should_update_child(c, to)) {
diff --git a/block/io.c b/block/io.c
index aee6e70c1496..571ff8c6493a 100644
--- a/block/io.c
+++ b/block/io.c
@@ -81,7 +81,7 @@ static void bdrv_parent_drained_end(BlockDriverState *bs, BdrvChild *ignore)
     }
 }
 
-bool bdrv_parent_drained_poll_single(BdrvChild *c)
+static bool bdrv_parent_drained_poll_single(BdrvChild *c)
 {
     if (c->klass->drained_poll) {
         return c->klass->drained_poll(c);
diff --git a/include/block/block-io.h b/include/block/block-io.h
index 75d043204355..0e0cd1249705 100644
--- a/include/block/block-io.h
+++ b/include/block/block-io.h
@@ -313,14 +313,6 @@ bdrv_writev_vmstate(BlockDriverState *bs, QEMUIOVector *qiov, int64_t pos);
  */
 void bdrv_parent_drained_begin_single(BdrvChild *c, bool poll);
 
-/**
- * bdrv_parent_drained_poll_single:
- *
- * Returns true if there is any pending activity to cease before @c can be
- * called quiesced, false otherwise.
- */
-bool bdrv_parent_drained_poll_single(BdrvChild *c);
-
 /**
  * bdrv_parent_drained_end_single:
  *
diff --git a/tests/unit/test-bdrv-drain.c b/tests/unit/test-bdrv-drain.c
index 2686a8aceee1..172bc6debc23 100644
--- a/tests/unit/test-bdrv-drain.c
+++ b/tests/unit/test-bdrv-drain.c
@@ -1654,7 +1654,6 @@ static void test_drop_intermediate_poll(void)
 
 
 typedef struct BDRVReplaceTestState {
-    bool setup_completed;
     bool was_drained;
     bool was_undrained;
     bool has_read;
@@ -1739,10 +1738,6 @@ static void bdrv_replace_test_drain_begin(BlockDriverState *bs)
 {
     BDRVReplaceTestState *s = bs->opaque;
 
-    if (!s->setup_completed) {
-        return;
-    }
-
     if (!s->drain_count) {
         s->drain_co = qemu_coroutine_create(bdrv_replace_test_drain_co, bs);
         bdrv_inc_in_flight(bs);
@@ -1774,10 +1769,6 @@ static void bdrv_replace_test_drain_end(BlockDriverState *bs)
 {
     BDRVReplaceTestState *s = bs->opaque;
 
-    if (!s->setup_completed) {
-        return;
-    }
-
     g_assert(s->drain_count > 0);
     if (!--s->drain_count) {
         s->was_undrained = true;
@@ -1876,7 +1867,6 @@ static void do_test_replace_child_mid_drain(int old_drain_count,
     bdrv_ref(old_child_bs);
     bdrv_attach_child(parent_bs, old_child_bs, "child", &child_of_bds,
                       BDRV_CHILD_COW, &error_abort);
-    parent_s->setup_completed = true;
 
     for (i = 0; i < old_drain_count; i++) {
         bdrv_drained_begin(old_child_bs);
-- 
2.38.1