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 <vsement...@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 <from...@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