kref: Implement kref_get_unless_zero v3
This function is intended to simplify locking around refcounting for objects that can be looked up from a lookup structure, and which are removed from that lookup structure in the object destructor. Operations on such objects require at least a read lock around lookup + kref_get, and a write lock around kref_put + remove from lookup structure. Furthermore, RCU implementations become extremely tricky. With a lookup followed by a kref_get_unless_zero *with return value check* locking in the kref_put path can be deferred to the actual removal from the lookup structure and RCU lookups become trivial. v2: Formatting fixes. v3: Invert the return value. Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com> Signed-off-by: Dave Airlie <airlied@redhat.com>
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@ -111,4 +111,25 @@ static inline int kref_put_mutex(struct kref *kref,
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}
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return 0;
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}
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/**
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* kref_get_unless_zero - Increment refcount for object unless it is zero.
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* @kref: object.
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*
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* Return non-zero if the increment succeeded. Otherwise return 0.
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*
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* This function is intended to simplify locking around refcounting for
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* objects that can be looked up from a lookup structure, and which are
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* removed from that lookup structure in the object destructor.
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* Operations on such objects require at least a read lock around
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* lookup + kref_get, and a write lock around kref_put + remove from lookup
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* structure. Furthermore, RCU implementations become extremely tricky.
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* With a lookup followed by a kref_get_unless_zero *with return value check*
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* locking in the kref_put path can be deferred to the actual removal from
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* the lookup structure and RCU lookups become trivial.
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*/
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static inline int __must_check kref_get_unless_zero(struct kref *kref)
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{
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return atomic_add_unless(&kref->refcount, 1, 0);
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}
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#endif /* _KREF_H_ */
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