eventfd: Add refcounting

[dumbbell]

Summary

An eventfd file descriptor can be used by drivers such as DRM drivers through linuxkpi. A driver can hold a reference to such a file regardless of the fact it is used by userland or not.

This patch introduces a refcount in struct eventfd_ctx, plus the eventfd_get() and eventfd_put() functions to acquire and release references. These functions will be used by DRM drivers for instance.

This structure is internal to sys/kern/sys_eventfd.c and not used anywhere else. Thus it is safe to add a field without breaking anything.

This is a requirement to the following patch to drm-kmod:
https://github.com/freebsd/drm-kmod/pull/358

Sponsored by: The FreeBSD Foundation

[markj]

This seems ok. It is kind of weird to have an eventfd that is not attached to a file descriptor, since there's nothing you can do with it. IMO it would be better for the drm drivers+linuxkpi to maintain a reference to the struct file instead, e.g., you could cheat and store a struct file * where drm stores a struct eventfd_ctx *, but that's a bit ugly too, and this is not a very intrusive diff.

sys/kern/sys_eventfd.c

103

It would be better to make it an unsigned int, that's the type used by the refcount(9) interface.