block/write-threshold: don't use write notifiers

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

Let's better special-case write-threshold and drop write notifiers at
all. (Actually, write-threshold is special-cased anyway, as the only
user of write-notifiers)

So, create a new direct interface for bdrv_co_write_req_prepare() and
drop all write-notifier related logic from write-threshold.c.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20210506090621.11848-2-vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
[mreitz: Adjusted comment as per Eric's suggestion]
Signed-off-by: Max Reitz <mreitz@redhat.com>

block/io.c

@@ -30,6 +30,7 @@
 #include "block/blockjob_int.h"
 #include "block/block_int.h"
 #include "block/coroutines.h"
+#include "block/write-threshold.h"
 #include "qemu/cutils.h"
 #include "qapi/error.h"
 #include "qemu/error-report.h"
@@ -2008,8 +2009,8 @@ 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);
+        bdrv_write_threshold_check_write(bs, offset, bytes);
+        return 0;
     case BDRV_TRACKED_TRUNCATE:
         assert(child->perm & BLK_PERM_RESIZE);
         return 0;

block/write-threshold.c

@@ -29,14 +29,6 @@ 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)
 {
@@ -51,55 +43,9 @@ uint64_t bdrv_write_threshold_exceeded(const BlockDriverState *bs,
     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;
-}
-
 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 */
-    }
+    bs->write_threshold_offset = threshold_bytes;
 }
 
 void qmp_block_set_write_threshold(const char *node_name,
@@ -122,3 +68,17 @@ void qmp_block_set_write_threshold(const char *node_name,
 
     aio_context_release(aio_context);
 }
+
+void bdrv_write_threshold_check_write(BlockDriverState *bs, int64_t offset,
+                                      int64_t bytes)
+{
+    int64_t end = offset + bytes;
+    uint64_t wtr = bs->write_threshold_offset;
+
+    if (wtr > 0 && end > wtr) {
+        qapi_event_send_block_write_threshold(bs->node_name, end - wtr, wtr);
+
+        /* autodisable to avoid flooding the monitor */
+        bdrv_write_threshold_set(bs, 0);
+    }
+}

include/block/block_int.h

@@ -959,7 +959,6 @@ struct BlockDriverState {
 
     /* 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

include/block/write-threshold.h

@@ -59,4 +59,13 @@ bool bdrv_write_threshold_is_set(const BlockDriverState *bs);
 uint64_t bdrv_write_threshold_exceeded(const BlockDriverState *bs,
                                        const BdrvTrackedRequest *req);
 
+/*
+ * bdrv_write_threshold_check_write
+ *
+ * Check whether the specified request exceeds the write threshold.
+ * If so, send a corresponding event and disable write threshold checking.
+ */
+void bdrv_write_threshold_check_write(BlockDriverState *bs, int64_t offset,
+                                      int64_t bytes);
+
 #endif