[PATCH] block: simplify write-threshold and drop write notifiers

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

write-notifiers are used only for write-threshold. New code for such
purpose should create filters.

Let's handle write-threshold simply in generic code and drop write
notifiers at all.

Also move part of write-threshold API that is used only for testing to
the test.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
---

I agree that this could be split into 2-3 parts and not combining
everything into one. But I'm tired now. I can send v2 if needed, so
consider it as RFC. Or take as is if you think it's not too much-in-one.

I also suggest this as a prepartion (and partly substitution) for
"[PATCH v2 0/8] Block layer thread-safety, continued"

 include/block/block_int.h         | 12 -----
 include/block/write-threshold.h   | 24 ---------
 block.c                           |  1 -
 block/io.c                        | 21 +++++---
 block/write-threshold.c           | 87 ++-----------------------------
 tests/unit/test-write-threshold.c | 38 ++++++++++++++
 6 files changed, 54 insertions(+), 129 deletions(-)

diff --git a/include/block/block_int.h b/include/block/block_int.h
index 88e4111939..50af58af75 100644
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -957,12 +957,8 @@ struct BlockDriverState {
      */
     int64_t total_sectors;
 
-    /* Callback before write request is processed */
-    NotifierWithReturnList before_write_notifiers;
-
     /* threshold limit for writes, in bytes. "High water mark". */
     uint64_t write_threshold_offset;
-    NotifierWithReturn write_threshold_notifier;
 
     /* Writing to the list requires the BQL _and_ the dirty_bitmap_mutex.
      * Reading from the list can be done with either the BQL or the
@@ -1087,14 +1083,6 @@ void bdrv_parse_filename_strip_prefix(const char *filename, const char *prefix,
 bool bdrv_backing_overridden(BlockDriverState *bs);
 
 
-/**
- * bdrv_add_before_write_notifier:
- *
- * Register a callback that is invoked before write requests are processed but
- * after any throttling or waiting for overlapping requests.
- */
-void bdrv_add_before_write_notifier(BlockDriverState *bs,
-                                    NotifierWithReturn *notifier);
 
 /**
  * bdrv_add_aio_context_notifier:
diff --git a/include/block/write-threshold.h b/include/block/write-threshold.h
index c646f267a4..23e1bfc123 100644
--- a/include/block/write-threshold.h
+++ b/include/block/write-threshold.h
@@ -35,28 +35,4 @@ void bdrv_write_threshold_set(BlockDriverState *bs, uint64_t threshold_bytes);
  */
 uint64_t bdrv_write_threshold_get(const BlockDriverState *bs);
 
-/*
- * bdrv_write_threshold_is_set
- *
- * Tell if a write threshold is set for a given BDS.
- */
-bool bdrv_write_threshold_is_set(const BlockDriverState *bs);
-
-/*
- * bdrv_write_threshold_exceeded
- *
- * Return the extent of a write request that exceeded the threshold,
- * or zero if the request is below the threshold.
- * Return zero also if the threshold was not set.
- *
- * NOTE: here we assume the following holds for each request this code
- * deals with:
- *
- * assert((req->offset + req->bytes) <= UINT64_MAX)
- *
- * Please not there is *not* an actual C assert().
- */
-uint64_t bdrv_write_threshold_exceeded(const BlockDriverState *bs,
-                                       const BdrvTrackedRequest *req);
-
 #endif
diff --git a/block.c b/block.c
index c5b887cec1..001453105e 100644
--- a/block.c
+++ b/block.c
@@ -381,7 +381,6 @@ BlockDriverState *bdrv_new(void)
     for (i = 0; i < BLOCK_OP_TYPE_MAX; i++) {
         QLIST_INIT(&bs->op_blockers[i]);
     }
-    notifier_with_return_list_init(&bs->before_write_notifiers);
     qemu_co_mutex_init(&bs->reqs_lock);
     qemu_mutex_init(&bs->dirty_bitmap_mutex);
     bs->refcnt = 1;
diff --git a/block/io.c b/block/io.c
index ca2dca3007..e0aa775952 100644
--- a/block/io.c
+++ b/block/io.c
@@ -36,6 +36,8 @@
 #include "qemu/main-loop.h"
 #include "sysemu/replay.h"
 
+#include "qapi/qapi-events-block-core.h"
+
 /* Maximum bounce buffer for copy-on-read and write zeroes, in bytes */
 #define MAX_BOUNCE_BUFFER (32768 << BDRV_SECTOR_BITS)
 
@@ -1974,6 +1976,8 @@ bdrv_co_write_req_prepare(BdrvChild *child, int64_t offset, int64_t bytes,
            child->perm & BLK_PERM_RESIZE);
 
     switch (req->type) {
+        uint64_t write_threshold;
+
     case BDRV_TRACKED_WRITE:
     case BDRV_TRACKED_DISCARD:
         if (flags & BDRV_REQ_WRITE_UNCHANGED) {
@@ -1981,8 +1985,15 @@ bdrv_co_write_req_prepare(BdrvChild *child, int64_t offset, int64_t bytes,
         } else {
             assert(child->perm & BLK_PERM_WRITE);
         }
-        return notifier_with_return_list_notify(&bs->before_write_notifiers,
-                                                req);
+        write_threshold = qatomic_read(&bs->write_threshold_offset);
+        if (write_threshold > 0 && offset + bytes > write_threshold) {
+            qapi_event_send_block_write_threshold(
+                bs->node_name,
+                offset + bytes - write_threshold,
+                write_threshold);
+            qatomic_set(&bs->write_threshold_offset, 0);
+        }
+        return 0;
     case BDRV_TRACKED_TRUNCATE:
         assert(child->perm & BLK_PERM_RESIZE);
         return 0;
@@ -3137,12 +3148,6 @@ bool bdrv_qiov_is_aligned(BlockDriverState *bs, QEMUIOVector *qiov)
     return true;
 }
 
-void bdrv_add_before_write_notifier(BlockDriverState *bs,
-                                    NotifierWithReturn *notifier)
-{
-    notifier_with_return_list_add(&bs->before_write_notifiers, notifier);
-}
-
 void bdrv_io_plug(BlockDriverState *bs)
 {
     BdrvChild *child;
diff --git a/block/write-threshold.c b/block/write-threshold.c
index 85b78dc2a9..9bf4287c6e 100644
--- a/block/write-threshold.c
+++ b/block/write-threshold.c
@@ -21,104 +21,23 @@
 
 uint64_t bdrv_write_threshold_get(const BlockDriverState *bs)
 {
-    return bs->write_threshold_offset;
-}
-
-bool bdrv_write_threshold_is_set(const BlockDriverState *bs)
-{
-    return bs->write_threshold_offset > 0;
-}
-
-static void write_threshold_disable(BlockDriverState *bs)
-{
-    if (bdrv_write_threshold_is_set(bs)) {
-        notifier_with_return_remove(&bs->write_threshold_notifier);
-        bs->write_threshold_offset = 0;
-    }
-}
-
-uint64_t bdrv_write_threshold_exceeded(const BlockDriverState *bs,
-                                       const BdrvTrackedRequest *req)
-{
-    if (bdrv_write_threshold_is_set(bs)) {
-        if (req->offset > bs->write_threshold_offset) {
-            return (req->offset - bs->write_threshold_offset) + req->bytes;
-        }
-        if ((req->offset + req->bytes) > bs->write_threshold_offset) {
-            return (req->offset + req->bytes) - bs->write_threshold_offset;
-        }
-    }
-    return 0;
-}
-
-static int coroutine_fn before_write_notify(NotifierWithReturn *notifier,
-                                            void *opaque)
-{
-    BdrvTrackedRequest *req = opaque;
-    BlockDriverState *bs = req->bs;
-    uint64_t amount = 0;
-
-    amount = bdrv_write_threshold_exceeded(bs, req);
-    if (amount > 0) {
-        qapi_event_send_block_write_threshold(
-            bs->node_name,
-            amount,
-            bs->write_threshold_offset);
-
-        /* autodisable to avoid flooding the monitor */
-        write_threshold_disable(bs);
-    }
-
-    return 0; /* should always let other notifiers run */
-}
-
-static void write_threshold_register_notifier(BlockDriverState *bs)
-{
-    bs->write_threshold_notifier.notify = before_write_notify;
-    bdrv_add_before_write_notifier(bs, &bs->write_threshold_notifier);
-}
-
-static void write_threshold_update(BlockDriverState *bs,
-                                   int64_t threshold_bytes)
-{
-    bs->write_threshold_offset = threshold_bytes;
+    return qatomic_read(&bs->write_threshold_offset);
 }
 
 void bdrv_write_threshold_set(BlockDriverState *bs, uint64_t threshold_bytes)
 {
-    if (bdrv_write_threshold_is_set(bs)) {
-        if (threshold_bytes > 0) {
-            write_threshold_update(bs, threshold_bytes);
-        } else {
-            write_threshold_disable(bs);
-        }
-    } else {
-        if (threshold_bytes > 0) {
-            /* avoid multiple registration */
-            write_threshold_register_notifier(bs);
-            write_threshold_update(bs, threshold_bytes);
-        }
-        /* discard bogus disable request */
-    }
+    qatomic_set(&bs->write_threshold_offset, threshold_bytes);
 }
 
 void qmp_block_set_write_threshold(const char *node_name,
                                    uint64_t threshold_bytes,
                                    Error **errp)
 {
-    BlockDriverState *bs;
-    AioContext *aio_context;
-
-    bs = bdrv_find_node(node_name);
+    BlockDriverState *bs = bdrv_find_node(node_name);
     if (!bs) {
         error_setg(errp, "Device '%s' not found", node_name);
         return;
     }
 
-    aio_context = bdrv_get_aio_context(bs);
-    aio_context_acquire(aio_context);
-
     bdrv_write_threshold_set(bs, threshold_bytes);
-
-    aio_context_release(aio_context);
 }
diff --git a/tests/unit/test-write-threshold.c b/tests/unit/test-write-threshold.c
index fc1c45a2eb..c2f4cd20d7 100644
--- a/tests/unit/test-write-threshold.c
+++ b/tests/unit/test-write-threshold.c
@@ -12,6 +12,44 @@
 #include "block/write-threshold.h"
 
 
+/*
+ * bdrv_write_threshold_is_set
+ *
+ * Tell if a write threshold is set for a given BDS.
+ */
+static bool bdrv_write_threshold_is_set(const BlockDriverState *bs)
+{
+    return bs->write_threshold_offset > 0;
+}
+
+/*
+ * bdrv_write_threshold_exceeded
+ *
+ * Return the extent of a write request that exceeded the threshold,
+ * or zero if the request is below the threshold.
+ * Return zero also if the threshold was not set.
+ *
+ * NOTE: here we assume the following holds for each request this code
+ * deals with:
+ *
+ * assert((req->offset + req->bytes) <= UINT64_MAX)
+ *
+ * Please not there is *not* an actual C assert().
+ */
+static uint64_t bdrv_write_threshold_exceeded(const BlockDriverState *bs,
+                                              const BdrvTrackedRequest *req)
+{
+    if (bdrv_write_threshold_is_set(bs)) {
+        if (req->offset > bs->write_threshold_offset) {
+            return (req->offset - bs->write_threshold_offset) + req->bytes;
+        }
+        if ((req->offset + req->bytes) > bs->write_threshold_offset) {
+            return (req->offset + req->bytes) - bs->write_threshold_offset;
+        }
+    }
+    return 0;
+}
+
 static void test_threshold_not_set_on_init(void)
 {
     uint64_t res;
-- 
2.29.2