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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 in async way by scheduling work function, for avoiding ublk dev and io_uring file's release dependency - Reinitializes io->ref and io->task_registered_buffers to clean state This ensures the reference count state is clean when ublk_queue_reinit() is called, preventing the warning and potential use-after-free. Fixes:1f6540e2aa("ublk: zc register/unregister bvec") Fixes:1ceeedb597("ublk: optimize UBLK_IO_UNREGISTER_IO_BUF on daemon task") Fixes:8a8fe42d76("ublk: optimize UBLK_IO_REGISTER_IO_BUF on daemon task") Signed-off-by: Ming Lei <ming.lei@redhat.com> Link: https://lore.kernel.org/r/20250827121602.2619736-2-ming.lei@redhat.com Signed-off-by: Jens Axboe <axboe@kernel.dk>