block/graph-lock: fix missed wakeup in bdrv_graph_co_rdunlock()

tests/qemu-iotests/tests/iothreads-create reproduces the hang on master under `stress-ng --cpu $(nproc) --timeout 0`. The iotest's vm.run_job() times out and qemu stays permanently stuck in ppoll(timeout=-1) inside bdrv_graph_wrlock_drained -> blk_remove_bs during qemu_cleanup(). The timing window is narrow on modern bare-metal hardware and much wider in a VM guest; downstream trees that still use plain bdrv_graph_wrlock() in blk_remove_bs() hit it on the first iteration under the same stress. bdrv_graph_wrlock() zeroes has_writer around its AIO_WAIT_WHILE loop so that callbacks dispatched by aio_poll() can still take the read lock on the fast path. The rdunlock side, however, only kicks a waiting writer when has_writer is observed set; a reader that drops its lock inside the polling window silently returns and nothing ever wakes the writer: main thread iothread0 coroutine ----------- ------------------- bdrv_graph_wrlock: rdlock held, reader_count=1 bdrv_drain_all_begin_nopoll has_writer = 0 AIO_WAIT_WHILE_UNLOCKED( NULL, reader_count >= 1): num_waiters++ smp_mb aio_poll(main_ctx, true) --> bdrv_graph_co_rdunlock: (ppoll, blocked) reader_count-- -> 0 smp_mb read has_writer = 0 skip aio_wait_kick() return reader_count is now 0 and num_waiters is still 1, but no BH, fd or timer on the main AioContext will fire -- the only entity that could kick just decided it did not have to. Main stays in ppoll() holding BQL, so RCU, VCPUs and any iothread path that needs BQL stall behind it. The hang is final; no timeout, no forward progress, no recovery as there is no other source of wake up inside qemu_cleanup(). bdrv_drain_all_begin() does not close the race on its own: it quiesces in-flight I/O, but graph readers also include non-I/O coroutines (block-job cleanup, virtio-scsi polling) that drain does not evict. The bdrv_graph_wrlock_drained() wrapper narrows the window but does not eliminate it; every plain bdrv_graph_wrlock() site is exposed on the same basis. Drop the has_writer check in bdrv_graph_co_rdunlock() and call aio_wait_kick() unconditionally. The helper itself loads num_waiters atomically and only schedules a dummy BH when a waiter exists, so the change is a no-op on the no-writer path and closes the missed-wakeup on the writer path. Signed-off-by: Denis V. Lunev <den@openvz.org> Cc: Kevin Wolf <kwolf@redhat.com> Cc: Hanna Reitz <hreitz@redhat.com> Cc: Stefan Hajnoczi <stefanha@redhat.com> Cc: Fiona Ebner <f.ebner@proxmox.com> Message-ID: <20260424103917.248668-2-den@openvz.org> Reviewed-by: Kevin Wolf <kwolf@redhat.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>