3fa8ee5faf
commit 4b20db3de8 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>
52 lines
1.7 KiB
C
52 lines
1.7 KiB
C
/*
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* kref.h - library routines for handling generic reference counted objects
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*
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* Copyright (C) 2004 Greg Kroah-Hartman <greg@kroah.com>
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* Copyright (C) 2004 IBM Corp.
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*
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* based on kobject.h which was:
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* Copyright (C) 2002-2003 Patrick Mochel <mochel@osdl.org>
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* Copyright (C) 2002-2003 Open Source Development Labs
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*
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* This file is released under the GPLv2.
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*
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*/
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#ifndef _KREF_H_
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#define _KREF_H_
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#include <linux/types.h>
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#include <linux/atomic.h>
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struct kref {
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atomic_t refcount;
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};
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void kref_init(struct kref *kref);
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void kref_get(struct kref *kref);
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int kref_put(struct kref *kref, void (*release) (struct kref *kref));
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int kref_sub(struct kref *kref, unsigned int count,
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void (*release) (struct kref *kref));
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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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