On Sat, Aug 23, 2025 at 10:55 AM Ming Lei [off-list ref] wrote:

When running test_stress_04.sh, the following warning is triggered:

WARNING: CPU: 1 PID: 135 at drivers/block/ublk_drv.c:1933 ublk_ch_release+0x423/0x4b0 [ublk_drv]

This happens when the daemon is abruptly killed:

- some references may still be held, because registering IO buffer
doesn't grab ublk char device reference

OR

- io->task_registered_buffers won't be cleared because io buffer is
released from non-daemon context

For zero-copy and auto buffer register modes, I/O reference crosses
syscalls, so IO reference may not be dropped naturally when ublk server is
killed abruptly. However, when releasing io_uring context, it is guaranteed
that the reference is dropped finally, see io_sqe_buffers_unregister() from
io_ring_ctx_free().

Fix this by adding ublk_drain_io_references() that:
- Waits for active I/O references dropped
- Reinitializes io->ref and io->task_registered_buffers to clean state

This way won't hang because releasing ublk char device doesn't depend on
unregistering sqe buffers in do_exit().

Actually io_uring file close may rely on ublk char dev close, so still has hang
risk, will send V2 for addressing it.

Thanks,