[PATCH 10/13] block: Call drain callbacks only once

[Kevin Wolf]

We only need to call both the BlockDriver's callback and the parent
callbacks when going from undrained to drained or vice versa. A second
drain section doesn't make a difference for the driver or the parent,
they weren't supposed to send new requests before and after the second
drain.

One thing that gets in the way is the 'ignore_bds_parents' parameter in
bdrv_do_drained_begin_quiesce() and bdrv_do_drained_end(): If it is true
for the first drain, bs->quiesce_counter will be non-zero, but the
parent callbacks still haven't been called, so a second drain where it
is false would still have to call them.

Instead of keeping track of this, let's just get rid of the parameter.
It was introduced in commit 6cd5c9d7b2d as an optimisation so that
during bdrv_drain_all(), we wouldn't recursively drain all parents up to
the root for each node, resulting in quadratic complexity. As it happens,
calling the callbacks only once solves the same problem, so as of this
patch, we'll still have O(n) complexity and ignore_bds_parents is not
needed any more.

This patch only ignores the 'ignore_bds_parents' parameter. It will be
removed in a separate patch.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
---
 block.c                      | 13 ++++++-------
 block/io.c                   | 24 +++++++++++++-----------
 tests/unit/test-bdrv-drain.c | 16 ++++++++++------
 3 files changed, 29 insertions(+), 24 deletions(-)

diff --git a/block.c b/block.c
index 9d082631d9..8878586f6e 100644
--- a/block.c
+++ b/block.c
@@ -2816,7 +2816,6 @@ static void bdrv_replace_child_noperm(BdrvChild *child,
 {
     BlockDriverState *old_bs = child->bs;
     int new_bs_quiesce_counter;
-    int drain_saldo;
 
     assert(!child->frozen);
     assert(old_bs != new_bs);
@@ -2827,15 +2826,13 @@ static void bdrv_replace_child_noperm(BdrvChild *child,
     }
 
     new_bs_quiesce_counter = (new_bs ? new_bs->quiesce_counter : 0);
-    drain_saldo = new_bs_quiesce_counter - child->parent_quiesce_counter;
 
     /*
      * If the new child node is drained but the old one was not, flush
      * all outstanding requests to the old child node.
      */
-    while (drain_saldo > 0 && child->klass->drained_begin) {
+    if (new_bs_quiesce_counter && !child->parent_quiesce_counter) {
         bdrv_parent_drained_begin_single(child, true);
-        drain_saldo--;
     }
 
     if (old_bs) {
@@ -2859,7 +2856,6 @@ static void bdrv_replace_child_noperm(BdrvChild *child,
          * more often.
          */
         assert(new_bs->quiesce_counter <= new_bs_quiesce_counter);
-        drain_saldo += new_bs->quiesce_counter - new_bs_quiesce_counter;
 
         if (child->klass->attach) {
             child->klass->attach(child);
@@ -2869,10 +2865,13 @@ static void bdrv_replace_child_noperm(BdrvChild *child,
     /*
      * 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.
      */
-    while (drain_saldo < 0 && child->klass->drained_end) {
+    new_bs_quiesce_counter = (new_bs ? new_bs->quiesce_counter : 0);
+    if (!new_bs_quiesce_counter && child->parent_quiesce_counter) {
         bdrv_parent_drained_end_single(child);
-        drain_saldo++;
     }
 }
 
diff --git a/block/io.c b/block/io.c
index 870a25d7a5..87c7a92f15 100644
--- a/block/io.c
+++ b/block/io.c
@@ -62,7 +62,7 @@ void bdrv_parent_drained_end_single(BdrvChild *c)
 {
     IO_OR_GS_CODE();
 
-    assert(c->parent_quiesce_counter > 0);
+    assert(c->parent_quiesce_counter == 1);
     c->parent_quiesce_counter--;
     if (c->klass->drained_end) {
         c->klass->drained_end(c);
@@ -109,6 +109,7 @@ static bool bdrv_parent_drained_poll(BlockDriverState *bs, 
BdrvChild *ignore,
 void bdrv_parent_drained_begin_single(BdrvChild *c, bool poll)
 {
     IO_OR_GS_CODE();
+    assert(c->parent_quiesce_counter == 0);
     c->parent_quiesce_counter++;
     if (c->klass->drained_begin) {
         c->klass->drained_begin(c);
@@ -352,16 +353,16 @@ void bdrv_do_drained_begin_quiesce(BlockDriverState *bs,
                                    BdrvChild *parent, bool ignore_bds_parents)
 {
     IO_OR_GS_CODE();
-    assert(!qemu_in_coroutine());
 
     /* Stop things in parent-to-child order */
     if (qatomic_fetch_inc(&bs->quiesce_counter) == 0) {
         aio_disable_external(bdrv_get_aio_context(bs));
-    }
 
-    bdrv_parent_drained_begin(bs, parent, ignore_bds_parents);
-    if (bs->drv && bs->drv->bdrv_drain_begin) {
-        bs->drv->bdrv_drain_begin(bs);
+        /* TODO Remove ignore_bds_parents, we don't consider it any more */
+        bdrv_parent_drained_begin(bs, parent, false);
+        if (bs->drv && bs->drv->bdrv_drain_begin) {
+            bs->drv->bdrv_drain_begin(bs);
+        }
     }
 }
 
@@ -412,13 +413,14 @@ static void bdrv_do_drained_end(BlockDriverState *bs, 
BdrvChild *parent,
     assert(bs->quiesce_counter > 0);
 
     /* Re-enable things in child-to-parent order */
-    if (bs->drv && bs->drv->bdrv_drain_end) {
-        bs->drv->bdrv_drain_end(bs);
-    }
-    bdrv_parent_drained_end(bs, parent, ignore_bds_parents);
-
     old_quiesce_counter = qatomic_fetch_dec(&bs->quiesce_counter);
     if (old_quiesce_counter == 1) {
+        if (bs->drv && bs->drv->bdrv_drain_end) {
+            bs->drv->bdrv_drain_end(bs);
+        }
+        /* TODO Remove ignore_bds_parents, we don't consider it any more */
+        bdrv_parent_drained_end(bs, parent, false);
+
         aio_enable_external(bdrv_get_aio_context(bs));
     }
 }
diff --git a/tests/unit/test-bdrv-drain.c b/tests/unit/test-bdrv-drain.c
index dda08de8db..172bc6debc 100644
--- a/tests/unit/test-bdrv-drain.c
+++ b/tests/unit/test-bdrv-drain.c
@@ -296,7 +296,11 @@ static void test_quiesce_common(enum drain_type 
drain_type, bool recursive)
 
     do_drain_begin(drain_type, bs);
 
-    g_assert_cmpint(bs->quiesce_counter, ==, 1);
+    if (drain_type == BDRV_DRAIN_ALL) {
+        g_assert_cmpint(bs->quiesce_counter, ==, 2);
+    } else {
+        g_assert_cmpint(bs->quiesce_counter, ==, 1);
+    }
     g_assert_cmpint(backing->quiesce_counter, ==, !!recursive);
 
     do_drain_end(drain_type, bs);
@@ -348,8 +352,8 @@ static void test_nested(void)
 
     for (outer = 0; outer < DRAIN_TYPE_MAX; outer++) {
         for (inner = 0; inner < DRAIN_TYPE_MAX; inner++) {
-            int backing_quiesce = (outer != BDRV_DRAIN) +
-                                  (inner != BDRV_DRAIN);
+            int backing_quiesce = (outer == BDRV_DRAIN_ALL) +
+                                  (inner == BDRV_DRAIN_ALL);
 
             g_assert_cmpint(bs->quiesce_counter, ==, 0);
             g_assert_cmpint(backing->quiesce_counter, ==, 0);
@@ -359,10 +363,10 @@ static void test_nested(void)
             do_drain_begin(outer, bs);
             do_drain_begin(inner, bs);
 
-            g_assert_cmpint(bs->quiesce_counter, ==, 2);
+            g_assert_cmpint(bs->quiesce_counter, ==, 2 + !!backing_quiesce);
             g_assert_cmpint(backing->quiesce_counter, ==, backing_quiesce);
-            g_assert_cmpint(s->drain_count, ==, 2);
-            g_assert_cmpint(backing_s->drain_count, ==, backing_quiesce);
+            g_assert_cmpint(s->drain_count, ==, 1);
+            g_assert_cmpint(backing_s->drain_count, ==, !!backing_quiesce);
 
             do_drain_end(inner, bs);
             do_drain_end(outer, bs);
-- 
2.38.1