New synchronization mechanism - localcount(9)


May 03, 2017 posted by Paul Goyette

Coming soon we have a new set of kernel synchronization routines - localcount(9) - which provide a medium-weight reference-counting mechanism. From the manual page, "During normal operations, localcounts do not need the interprocessor synchronization associated with atomic_ops(3) atomic memory operations, and (unlike psref(9)) localcount references can be held across sleeps and can migrate between CPUs. Draining a localcount requires more expensive interprocessor synchronization than atomic_ops(3) (similar to psref(9)). And localcount references require eight bytes of memory per object per-CPU, significantly more than atomic_ops(3) and almost always more than psref(9)."

We'll be adding localcount(9) reference counting to the device driver cdevsw and bdevsw structures, to ensure that a (modular) device driver cannot be removed while it is active. Modular drivers with initializers for these structures need to be modified to initialize their localcount members, using the DEVSW_MODULE_INIT macro (this change is mandatory for all loadable drivers). To take advantage of the reference counting, the drivers also need to replace all calls to bdevsw_lookup() and cdevsw_lookup() with bdevsw_lookup_acquire() and cdevsw_lookup_acquire() respectively, and then release the reference using bdevsw_release() and cdevsw_release().

We'll also be using localcount(9) to provide reference-counting of individual device units, to prevent a unit from being destroyed while it is active. To implement device unit reference-counting, all calls to device_lookup(), device_find_by_driver_unit(), and device_lookup_private() need to be replaced by their corresponding *_acquire() variant; when the caller is finished using the device, it must release the reference using device_release().

More details and examples can be seen by examining the prg-localcount2 branch in cvs including the new localcount(9) manual page!

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