 Unwinding a failed character-device registration

Unwinding a failed character-device registration releases resources acquired before an initialization failure while preserving the lifetime of anything that concurrent users can still access. In Linux, reserving device numbers, publishing the character-device object and creating userspace-visible names are distinct steps. The cleanup path must know which steps actually succeeded.

Consider an original abstract initialization sequence: reserve a number range, initialize a character-device object, publish it, then finish a later setup step. The following table tracks successful acquisitions; it is not a complete driver implementation.

alloc_chrdev_region reserves a range and reports failure through its return value. cdev_init initializes the object with its operations; cdev_add publishes it and makes it live immediately. unregister_chrdev_region releases a successful number reservation. Pair each release with the acquisition that actually succeeded, using the same first number and count.^[ref]

For a concrete bookkeeping example, suppose a successful reservation contains four interfaces starting at returned pair $(M,20)$. Failure before publication requires releasing that four-number range. Releasing only three leaves part of the reservation behind; releasing a guessed starting major or an unacquired range is a different error. The returned major from dynamic character-device number allocation must be retained rather than reconstructed from a pathname.

Publication changes the lifetime problem. cdev_del prevents new opens, but existing open file descriptors can still invoke their file operations. Therefore removing the device from lookup is not a proof that its backing state can immediately be freed. Character-device lifetime after deletion must account for the references and callbacks already in progress.^[ref]

The combined cdev_device_add helper has an especially important failure contract: callers must assume userspace could open the device and call operations even if the helper returns failure. A blanket error handler that immediately frees all backing state would violate that contract.^[ref] Design ownership and reference handling around the documented publication boundary, then release each owned resource once. The general lesson is that reverse acquisition order must be combined with concurrency-aware lifetime management.