kref_get_not_zero() [LWN.net]

kref_get_not_zero()

Posted Jun 9, 2009 20:42 UTC (Tue) by neilbrown (subscriber, #359)
In reply to: kref_get_not_zero() by abatters
Parent article: Linux kernel design patterns - part 1

Thanks for that reference.

I cannot comment on the code under the microscope in that thread as I am not familiar with it at all. However your observation that kref_get_not_zero() can be safe when used under a lock that excludes the object from being freed is one that I completely agree with. It is therefore tempting to call the function kref_get_locked() to match the inode_get_locked that already exists.

However it can be valid to use atomic_inc_not_zero() when not holding a lock, as in the example in a previous thread. In that case it is safe not because a lock is held, but because a secondary reference is held (s_count).

Either way, the complete pattern description for kref would need to spell out how and when to use kref_get_not_zero() (or whatever it gets called).

I must say that Greg's 'ick' in the reply to your linked post surprised me. kref_get_not_zero is (in my view) much less 'ick' than kref_set which is part of the current kref API (exercise: find all 3 places that use kref_set and replace them with calls to other parts of the api).

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