Am 12.12.2022 um 13:59 hat Paolo Bonzini geschrieben: > 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.
This is true in the initial drain phase while we're still polling. But generally this is not only about avoiding a continuous stream of requests, but about making sure that absolutely no new requests come in while a node is drained. > + */ > + if (!blk->disable_request_queuing && > qatomic_read(&blk->quiesce_counter)) { So if no other requests were pending and we didn't even call aio_poll() because the AIO_WAIT_WHILE() condition was false from the start, could it be that bdrv_drained_begin() has already returned on the thread that drains, but another thread still sees the old value here? Starting a new request after bdrv_drained_begin() has returned would be a bug. Kevin