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[PATCH v3 5/8] block/write-threshold: don't use aio context lock
From: |
Vladimir Sementsov-Ogievskiy |
Subject: |
[PATCH v3 5/8] block/write-threshold: don't use aio context lock |
Date: |
Thu, 6 May 2021 12:06:18 +0300 |
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 <vsementsov@virtuozzo.com>
---
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 <fromani@redhat.com>
@@ -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 let's send event */
+ qapi_event_send_block_write_threshold(bs->node_name, end - wtr, wtr);
}
--
2.29.2
- [PATCH v3 0/8] block: refactor write threshold, Vladimir Sementsov-Ogievskiy, 2021/05/06
- [PATCH v3 2/8] block: drop write notifiers, Vladimir Sementsov-Ogievskiy, 2021/05/06
- [PATCH v3 5/8] block/write-threshold: don't use aio context lock,
Vladimir Sementsov-Ogievskiy <=
- [PATCH v3 1/8] block/write-threshold: don't use write notifiers, Vladimir Sementsov-Ogievskiy, 2021/05/06
- [PATCH v3 3/8] test-write-threshold: rewrite test_threshold_(not_)trigger tests, Vladimir Sementsov-Ogievskiy, 2021/05/06
- [PATCH v3 8/8] write-threshold: deal with includes, Vladimir Sementsov-Ogievskiy, 2021/05/06
- [PATCH v3 4/8] block/write-threshold: drop extra APIs, Vladimir Sementsov-Ogievskiy, 2021/05/06
- [PATCH v3 6/8] test-write-threshold: drop extra tests, Vladimir Sementsov-Ogievskiy, 2021/05/06
- [PATCH v3 7/8] test-write-threshold: drop extra TestStruct structure, Vladimir Sementsov-Ogievskiy, 2021/05/06