Instead of relying on aio context lock, let's make use of atomic
operations.
The tricky place is bdrv_write_threshold_check_write(): we want
atomically unset bs->write_threshold_offset iff
offset + bytes > bs->write_threshold_offset
We don't have such atomic operation, so let's go in a loop:
1. fetch wtr atomically
2. if condition satisfied, try cmpxchg (if not satisfied, we are done,
don't send event)
3. if cmpxchg succeeded, we are done (send event), else go to [1]
Signed-off-by: Vladimir Sementsov-Ogievskiy
---
include/block/block_int.h | 6 +-
include/block/write-threshold.h | 6 ++
block/write-threshold.c | 34 +
3 files changed, 29 insertions(+), 17 deletions(-)
diff --git a/include/block/block_int.h b/include/block/block_int.h
index eab352f363..e3f3d79f5b 100644
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -954,7 +954,11 @@ struct BlockDriverState {
*/
int64_t total_sectors;
-/* threshold limit for writes, in bytes. "High water mark". */
+/*
+ * Threshold limit for writes, in bytes. "High water mark".
+ * Don't access directly, use bdrw_write_threshold* interface.
+ * Protected by atomic access, no lock is needed.
+ */
uint64_t write_threshold_offset;
/* Writing to the list requires the BQL _and_ the dirty_bitmap_mutex.
diff --git a/include/block/write-threshold.h b/include/block/write-threshold.h
index c60b9954cd..28c35a1c05 100644
--- a/include/block/write-threshold.h
+++ b/include/block/write-threshold.h
@@ -24,6 +24,8 @@
* To be used with thin-provisioned block devices.
*
* Use threshold_bytes == 0 to disable.
+ *
+ * Function is thread-safe, no lock is needed.
*/
void bdrv_write_threshold_set(BlockDriverState *bs, uint64_t threshold_bytes);
@@ -32,6 +34,8 @@ void bdrv_write_threshold_set(BlockDriverState *bs, uint64_t
threshold_bytes);
*
* Get the configured write threshold, in bytes.
* Zero means no threshold configured.
+ *
+ * Function is thread-safe, no lock is needed.
*/
uint64_t bdrv_write_threshold_get(const BlockDriverState *bs);
@@ -40,6 +44,8 @@ uint64_t bdrv_write_threshold_get(const BlockDriverState *bs);
*
* Check whether the specified request exceeds the write threshold.
* If it is, send corresponding event and disable write threshold checking.
+ *
+ * Function is thread-safe, no lock is needed.
*/
void bdrv_write_threshold_check_write(BlockDriverState *bs, int64_t offset,
int64_t bytes);
diff --git a/block/write-threshold.c b/block/write-threshold.c
index 65a6acd142..8b46bb9a75 100644
--- a/block/write-threshold.c
+++ b/block/write-threshold.c
@@ -2,6 +2,7 @@
* QEMU System Emulator block write threshold notification
*
* Copyright Red Hat, Inc. 2014
+ * Copyright (c) 2021 Virtuozzo International GmbH.
*
* Authors:
* Francesco Romani
@@ -14,6 +15,7 @@
#include "block/block_int.h"
#include "qemu/coroutine.h"
#include "block/write-threshold.h"
+#include "qemu/atomic.h"
#include "qemu/notify.h"
#include "qapi/error.h"
#include "qapi/qapi-commands-block-core.h"
@@ -21,45 +23,45 @@
uint64_t bdrv_write_threshold_get(const BlockDriverState *bs)
{
-return bs->write_threshold_offset;
+return qatomic_read(&bs->write_threshold_offset);
}
void bdrv_write_threshold_set(BlockDriverState *bs, uint64_t threshold_bytes)
{
-bs->write_threshold_offset = threshold_bytes;
+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);
}
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;
+uint64_t wtr;
-if (wtr > 0 && end > wtr) {
-qapi_event_send_block_write_threshold(bs->node_name, end - wtr, wtr);
+retry:
+wtr = bdrv_write_threshold_get(bs);
+if (wtr == 0 || wtr >= end) {
+return;
+}
-/* autodisable to avoid flooding the monitor */
-bdrv_write_threshold_set(bs, 0);
+/* autodisable to avoid flooding the monitor */
+if (qatomic_cmpxchg(&bs->write_threshold_offset, wtr, 0) != wtr) {
+/* bs->write_threshold_offset changed in parallel */
+goto retry;
}
+
+/* We have cleared bs->write_threshold_offset, so