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> --- block/write-threshold.c | 32 +++++++++++++++++--------------- 1 file changed, 17 insertions(+), 15 deletions(-) diff --git a/block/write-threshold.c b/block/write-threshold.c index ae54ee05dc..fbf4e6f5c4 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> @@ -21,43 +22,44 @@ 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); - bdrv_write_threshold_set(bs, 0); +retry: + wtr = bdrv_write_threshold_get(bs); + if (wtr == 0 || wtr >= end) { + return; } + + 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