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: abe54c1603 ("selftests: ublk: kublk: decouple ublk_queues from ublk server threads")
Signed-off-by: Ming Lei <tom.leiming@gmail.com>
Link: https://patch.msgid.link/20260513101941.1373998-1-tom.leiming@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
87d0740b7c