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block: make BlockBackend->quiesce_counter atomic 

The main loop thread increments/decrements BlockBackend->quiesce_counter
when drained sections begin/end. The counter is read in the I/O code
path. Therefore this field is used to communicate between threads
without a lock.

Acquire/release are not necessary because the BlockBackend->in_flight
counter already uses sequentially consistent accesses and running I/O
requests hold that counter when blk_wait_while_drained() is called.
qatomic_read() can be used.

Use qatomic_fetch_inc()/qatomic_fetch_dec() for modifications even
though sequentially consistent atomic accesses are not strictly required
here. They are, however, nicer to read than multiple calls to
qatomic_read() and qatomic_set(). Since beginning and ending drain is
not a hot path the extra cost doesn't matter.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20230307210427.269214-2-stefanha@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
This commit is contained in:
Stefan Hajnoczi 2023-03-07 16:04:25 -05:00 committed by Kevin Wolf
parent ac5f7bf8e2
commit c4d5bf99b7
1 changed files with 7 additions and 7 deletions
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14
block/block-backend.c
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@ -80,7 +80,7 @@ struct BlockBackend {
NotifierList remove_bs_notifiers, insert_bs_notifiers;
QLIST_HEAD(, BlockBackendAioNotifier) aio_notifiers;
int quiesce_counter;
int quiesce_counter; /* atomic: written under BQL, read by other threads */
CoQueue queued_requests;
bool disable_request_queuing;
@ -1057,7 +1057,7 @@ void blk_set_dev_ops(BlockBackend *blk, const BlockDevOps *ops,
blk->dev_opaque = opaque;
/* Are we currently quiesced? Should we enforce this right now? */
if (blk->quiesce_counter && ops && ops->drained_begin) {
if (qatomic_read(&blk->quiesce_counter) && ops && ops->drained_begin) {
ops->drained_begin(opaque);
}
}
@ -1271,7 +1271,7 @@ static void coroutine_fn blk_wait_while_drained(BlockBackend *blk)
{
assert(blk->in_flight > 0);
if (blk->quiesce_counter && !blk->disable_request_queuing) {
if (qatomic_read(&blk->quiesce_counter) && !blk->disable_request_queuing) {
blk_dec_in_flight(blk);
qemu_co_queue_wait(&blk->queued_requests, NULL);
blk_inc_in_flight(blk);
@ -2595,7 +2595,7 @@ static void blk_root_drained_begin(BdrvChild *child)
BlockBackend *blk = child->opaque;
ThrottleGroupMember *tgm = &blk->public.throttle_group_member;
if (++blk->quiesce_counter == 1) {
if (qatomic_fetch_inc(&blk->quiesce_counter) == 0) {
if (blk->dev_ops && blk->dev_ops->drained_begin) {
blk->dev_ops->drained_begin(blk->dev_opaque);
}
@ -2613,7 +2613,7 @@ static bool blk_root_drained_poll(BdrvChild *child)
{
BlockBackend *blk = child->opaque;
bool busy = false;
assert(blk->quiesce_counter);
assert(qatomic_read(&blk->quiesce_counter));
if (blk->dev_ops && blk->dev_ops->drained_poll) {
busy = blk->dev_ops->drained_poll(blk->dev_opaque);
@ -2624,12 +2624,12 @@ static bool blk_root_drained_poll(BdrvChild *child)
static void blk_root_drained_end(BdrvChild *child)
{
BlockBackend *blk = child->opaque;
assert(blk->quiesce_counter);
assert(qatomic_read(&blk->quiesce_counter));
assert(blk->public.throttle_group_member.io_limits_disabled);
qatomic_dec(&blk->public.throttle_group_member.io_limits_disabled);
if (--blk->quiesce_counter == 0) {
if (qatomic_fetch_dec(&blk->quiesce_counter) == 1) {
if (blk->dev_ops && blk->dev_ops->drained_end) {
blk->dev_ops->drained_end(blk->dev_opaque);
}