855498f671
the destroying cdev. Currently linux_destroy_dev() waits for the reference count on the linux cdev to drain, and each open file hold the reference. Practically it means that linux_destroy_dev() is blocked until all userspace processes that have the cdev open, exit. FreeBSD devfs does not have such problem, because device refcount only prevents freeing of the cdev memory, and separate 'active methods' counter blocks destroy_dev() until all threads leave the cdevsw methods. After that, attempts to enter cdevsw methods are refused with an error. Implement somewhat similar mechanism for LinuxKPI cdevs. Demote cdev refcount to only mean a hold on the linux cdev memory. Add sirefs count to track both number of threads inside the cdev methods, and for single-bit indicator that cdev is being destroyed. In the later case, the call is redirected to the dummy cdev. Reviewed by: markj Discussed with: hselasky Tested by: zeising MFC after: 1 week Sponsored by: Mellanox Technologies Differential revision: https://reviews.freebsd.org/D18606
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