Messages in this thread Patch in this message | | | From | Ben Hutchings <> | Date | Mon, 22 Apr 2013 15:25:59 +0100 | Subject | [40/75] kref: Implement kref_get_unless_zero v3 |
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3.2.44-rc1 review patch. If anyone has any objections, please let me know.
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From: Thomas Hellstrom <thellstrom@vmware.com>
commit 4b20db3de8dab005b07c74161cb041db8c5ff3a7 upstream.
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> [bwh: Backported to 3.2: - Adjust context - Add #include <linux/atomic.h>] Signed-off-by: Ben Hutchings <ben@decadent.org.uk> --- --- a/include/linux/kref.h +++ b/include/linux/kref.h @@ -16,6 +16,7 @@ #define _KREF_H_ #include <linux/types.h> +#include <linux/atomic.h> struct kref { atomic_t refcount; @@ -27,4 +28,24 @@ int kref_put(struct kref *kref, void (*r int kref_sub(struct kref *kref, unsigned int count, void (*release) (struct kref *kref)); +/** + * kref_get_unless_zero - Increment refcount for object unless it is zero. + * @kref: object. + * + * Return non-zero if the increment succeeded. Otherwise return 0. + * + * 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. + */ +static inline int __must_check kref_get_unless_zero(struct kref *kref) +{ + return atomic_add_unless(&kref->refcount, 1, 0); +} #endif /* _KREF_H_ */
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