153 lines
4.1 KiB
C
153 lines
4.1 KiB
C
/* SPDX-License-Identifier: GPL-2.0 */
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#ifndef _TOOLS_LINUX_REFCOUNT_H
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#define _TOOLS_LINUX_REFCOUNT_H
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/*
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* Variant of atomic_t specialized for reference counts.
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*
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* The interface matches the atomic_t interface (to aid in porting) but only
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* provides the few functions one should use for reference counting.
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*
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* It differs in that the counter saturates at UINT_MAX and will not move once
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* there. This avoids wrapping the counter and causing 'spurious'
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* use-after-free issues.
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*
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* Memory ordering rules are slightly relaxed wrt regular atomic_t functions
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* and provide only what is strictly required for refcounts.
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*
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* The increments are fully relaxed; these will not provide ordering. The
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* rationale is that whatever is used to obtain the object we're increasing the
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* reference count on will provide the ordering. For locked data structures,
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* its the lock acquire, for RCU/lockless data structures its the dependent
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* load.
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*
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* Do note that inc_not_zero() provides a control dependency which will order
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* future stores against the inc, this ensures we'll never modify the object
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* if we did not in fact acquire a reference.
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*
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* The decrements will provide release order, such that all the prior loads and
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* stores will be issued before, it also provides a control dependency, which
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* will order us against the subsequent free().
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*
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* The control dependency is against the load of the cmpxchg (ll/sc) that
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* succeeded. This means the stores aren't fully ordered, but this is fine
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* because the 1->0 transition indicates no concurrency.
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*
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* Note that the allocator is responsible for ordering things between free()
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* and alloc().
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*
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*/
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#include <linux/atomic.h>
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#include <linux/kernel.h>
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#ifdef NDEBUG
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#define REFCOUNT_WARN(cond, str) (void)(cond)
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#define __refcount_check
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#else
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#define REFCOUNT_WARN(cond, str) BUG_ON(cond)
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#define __refcount_check __must_check
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#endif
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typedef struct refcount_struct {
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atomic_t refs;
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} refcount_t;
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#define REFCOUNT_INIT(n) { .refs = ATOMIC_INIT(n), }
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static inline void refcount_set(refcount_t *r, unsigned int n)
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{
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atomic_set(&r->refs, n);
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}
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static inline unsigned int refcount_read(const refcount_t *r)
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{
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return atomic_read(&r->refs);
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}
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/*
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* Similar to atomic_inc_not_zero(), will saturate at UINT_MAX and WARN.
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*
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* Provides no memory ordering, it is assumed the caller has guaranteed the
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* object memory to be stable (RCU, etc.). It does provide a control dependency
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* and thereby orders future stores. See the comment on top.
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*/
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static inline __refcount_check
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bool refcount_inc_not_zero(refcount_t *r)
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{
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unsigned int old, new, val = atomic_read(&r->refs);
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for (;;) {
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new = val + 1;
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if (!val)
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return false;
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if (unlikely(!new))
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return true;
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old = atomic_cmpxchg_relaxed(&r->refs, val, new);
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if (old == val)
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break;
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val = old;
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}
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REFCOUNT_WARN(new == UINT_MAX, "refcount_t: saturated; leaking memory.\n");
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return true;
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}
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/*
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* Similar to atomic_inc(), will saturate at UINT_MAX and WARN.
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*
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* Provides no memory ordering, it is assumed the caller already has a
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* reference on the object, will WARN when this is not so.
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*/
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static inline void refcount_inc(refcount_t *r)
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{
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REFCOUNT_WARN(!refcount_inc_not_zero(r), "refcount_t: increment on 0; use-after-free.\n");
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}
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/*
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* Similar to atomic_dec_and_test(), it will WARN on underflow and fail to
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* decrement when saturated at UINT_MAX.
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*
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* Provides release memory ordering, such that prior loads and stores are done
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* before, and provides a control dependency such that free() must come after.
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* See the comment on top.
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*/
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static inline __refcount_check
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bool refcount_sub_and_test(unsigned int i, refcount_t *r)
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{
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unsigned int old, new, val = atomic_read(&r->refs);
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for (;;) {
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if (unlikely(val == UINT_MAX))
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return false;
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new = val - i;
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if (new > val) {
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REFCOUNT_WARN(new > val, "refcount_t: underflow; use-after-free.\n");
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return false;
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}
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old = atomic_cmpxchg_release(&r->refs, val, new);
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if (old == val)
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break;
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val = old;
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}
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return !new;
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}
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static inline __refcount_check
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bool refcount_dec_and_test(refcount_t *r)
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{
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return refcount_sub_and_test(1, r);
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}
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#endif /* _ATOMIC_LINUX_REFCOUNT_H */
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