Protect quiesce_counter and queued_requests with a QemuMutex, so that
they are protected from concurrent access in the main thread (for example
blk_root_drained_end() reached from bdrv_drain_all()) and in the iothread
(where any I/O operation will call blk_inc_in_flight()).
Signed-off-by: Paolo Bonzini <pbonz...@redhat.com>
---
block/block-backend.c | 44 +++++++++++++++++++++++++++++++++++--------
1 file changed, 36 insertions(+), 8 deletions(-)
diff --git a/block/block-backend.c b/block/block-backend.c
index 627d491d4155..fdf82f1f1599 100644
--- a/block/block-backend.c
+++ b/block/block-backend.c
@@ -23,6 +23,7 @@
#include "qapi/error.h"
#include "qapi/qapi-events-block.h"
#include "qemu/id.h"
+#include "qemu/thread.h"
#include "qemu/main-loop.h"
#include "qemu/option.h"
#include "trace.h"
@@ -85,6 +86,8 @@ struct BlockBackend {
NotifierList remove_bs_notifiers, insert_bs_notifiers;
QLIST_HEAD(, BlockBackendAioNotifier) aio_notifiers;
+ /* Protected by quiesce_lock. */
+ QemuMutex quiesce_lock;
int quiesce_counter;
CoQueue queued_requests;
@@ -372,6 +375,7 @@ BlockBackend *blk_new(AioContext *ctx, uint64_t perm,
uint64_t shared_perm)
block_acct_init(&blk->stats);
+ qemu_mutex_init(&blk->quiesce_lock);
qemu_co_queue_init(&blk->queued_requests);
notifier_list_init(&blk->remove_bs_notifiers);
notifier_list_init(&blk->insert_bs_notifiers);
@@ -490,6 +494,7 @@ static void blk_delete(BlockBackend *blk)
assert(QLIST_EMPTY(&blk->insert_bs_notifiers.notifiers));
assert(QLIST_EMPTY(&blk->aio_notifiers));
QTAILQ_REMOVE(&block_backends, blk, link);
+ qemu_mutex_destroy(&blk->quiesce_lock);
drive_info_del(blk->legacy_dinfo);
block_acct_cleanup(&blk->stats);
g_free(blk);
@@ -1451,11 +1456,25 @@ void blk_inc_in_flight(BlockBackend *blk)
{
IO_CODE();
qatomic_inc(&blk->in_flight);
- if (!blk->disable_request_queuing) {
- /* TODO: this is not thread-safe! */
+
+ /*
+ * Avoid a continuous stream of requests from AIO callbacks, which
+ * call a user-provided callback while blk->in_flight is elevated,
+ * if the BlockBackend is being quiesced.
+ *
+ * This initial test does not have to be perfect: a race might
+ * cause an extra I/O to be queued, but sooner or later a nonzero
+ * quiesce_counter will be observed.
+ */
+ if (!blk->disable_request_queuing && qatomic_read(&blk->quiesce_counter)) {
+ /*
+ * ... on the other hand, it is important that the final check and
+ * the wait are done under the lock, so that no wakeups are missed.
+ */
+ QEMU_LOCK_GUARD(&blk->quiesce_lock);
while (blk->quiesce_counter) {
qatomic_dec(&blk->in_flight);
- qemu_co_queue_wait(&blk->queued_requests, NULL);
+ qemu_co_queue_wait(&blk->queued_requests, &blk->quiesce_lock);
qatomic_inc(&blk->in_flight);
}
}
@@ -2542,7 +2561,8 @@ static void blk_root_drained_begin(BdrvChild *child)
BlockBackend *blk = child->opaque;
ThrottleGroupMember *tgm = &blk->public.throttle_group_member;
- if (++blk->quiesce_counter == 1) {
+ qatomic_set(&blk->quiesce_counter, blk->quiesce_counter + 1);
+ if (blk->quiesce_counter == 1) {
if (blk->dev_ops && blk->dev_ops->drained_begin) {
blk->dev_ops->drained_begin(blk->dev_opaque);
}
@@ -2560,6 +2580,7 @@ static bool blk_root_drained_poll(BdrvChild *child)
{
BlockBackend *blk = child->opaque;
bool busy = false;
+
assert(blk->quiesce_counter);
if (blk->dev_ops && blk->dev_ops->drained_poll) {
@@ -2576,14 +2597,21 @@ static void blk_root_drained_end(BdrvChild *child)
assert(blk->public.throttle_group_member.io_limits_disabled);
qatomic_dec(&blk->public.throttle_group_member.io_limits_disabled);
- if (--blk->quiesce_counter == 0) {
+ qemu_mutex_lock(&blk->quiesce_lock);
+ qatomic_set(&blk->quiesce_counter, blk->quiesce_counter - 1);
+ if (blk->quiesce_counter == 0) {
+ /* After this point, new I/O will not sleep on queued_requests. */
+ qemu_mutex_unlock(&blk->quiesce_lock);
+
if (blk->dev_ops && blk->dev_ops->drained_end) {
blk->dev_ops->drained_end(blk->dev_opaque);
}
- while (qemu_co_enter_next(&blk->queued_requests, NULL)) {
- /* Resume all queued requests */
- }
+
+ /* Now resume all queued requests */
+ qemu_mutex_lock(&blk->quiesce_lock);
+ qemu_co_enter_all(&blk->queued_requests, &blk->quiesce_lock);
}
+ qemu_mutex_unlock(&blk->quiesce_lock);
}
bool blk_register_buf(BlockBackend *blk, void *host, size_t size, Error **errp)
--
2.38.1
