dev->nthreads is derived from the user-requested queue count before the ADD command, but the kernel may reduce nr_hw_queues (capped to nr_cpu_ids). When the VM has fewer CPUs than requested queues, the daemon creates more handler threads than there are kernel queues. In non-batch mode, the extra threads access uninitialized queues (q_depth=0), submit zero io_uring SQEs, and block forever in io_cqring_wait. In batch mode, the extra threads cause similar hangs during device removal. In both cases, the stuck threads prevent the daemon from closing the char device, holding the last ublk_device reference and causing ublk_ctrl_del_dev() to hang in wait_event_interruptible(). Fix by capping dev->nthreads to the kernel-returned nr_hw_queues after the ADD command completes. per_io_tasks mode is excluded because threads interleave across all queues, so nthreads > nr_hw_queues is valid. Fixes: abe54c160346 ("selftests: ublk: kublk: decouple ublk_queues from ublk server threads") Signed-off-by: Ming Lei --- V2: - fix email address & signed-off-by tools/testing/selftests/ublk/kublk.c | 11 +++++++++++ 1 file changed, 11 insertions(+) diff --git a/tools/testing/selftests/ublk/kublk.c b/tools/testing/selftests/ublk/kublk.c index fbd9b1e7342a..0b23c09daea5 100644 --- a/tools/testing/selftests/ublk/kublk.c +++ b/tools/testing/selftests/ublk/kublk.c @@ -1735,6 +1735,17 @@ static int __cmd_dev_add(const struct dev_ctx *ctx) goto fail; } + /* + * The kernel may reduce nr_hw_queues (e.g. capped to nr_cpu_ids). + * Cap nthreads to the actual queue count to avoid creating extra + * handler threads that will hang during device removal. + * + * per_io_tasks mode is excluded: threads interleave across all + * queues so nthreads > nr_hw_queues is valid and intentional. + */ + if (!ctx->per_io_tasks && dev->nthreads > info->nr_hw_queues) + dev->nthreads = info->nr_hw_queues; + ret = ublk_start_daemon(ctx, dev); ublk_dbg(UBLK_DBG_DEV, "%s: daemon exit %d\n", __func__, ret); if (ret < 0) -- 2.53.0