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Linux kernel mirror (for testing) git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
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123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899100101102103104105106107108109110111112113114115116117118119120121122123124125126127128129130131132133134135136137138139140141142143144145146147148149150151152153154155156157158/* SPDX-License-Identifier: GPL-2.0 */#ifndef _TOOLS_LINUX_REFCOUNT_H#define _TOOLS_LINUX_REFCOUNT_H
/* * Variant of atomic_t specialized for reference counts. * * The interface matches the atomic_t interface (to aid in porting) but only * provides the few functions one should use for reference counting. * * It differs in that the counter saturates at UINT_MAX and will not move once * there. This avoids wrapping the counter and causing 'spurious' * use-after-free issues. * * Memory ordering rules are slightly relaxed wrt regular atomic_t functions * and provide only what is strictly required for refcounts. * * The increments are fully relaxed; these will not provide ordering. The * rationale is that whatever is used to obtain the object we're increasing the * reference count on will provide the ordering. For locked data structures, * its the lock acquire, for RCU/lockless data structures its the dependent * load. * * Do note that inc_not_zero() provides a control dependency which will order * future stores against the inc, this ensures we'll never modify the object * if we did not in fact acquire a reference. * * The decrements will provide release order, such that all the prior loads and * stores will be issued before, it also provides a control dependency, which * will order us against the subsequent free(). * * The control dependency is against the load of the cmpxchg (ll/sc) that * succeeded. This means the stores aren't fully ordered, but this is fine * because the 1->0 transition indicates no concurrency. * * Note that the allocator is responsible for ordering things between free() * and alloc(). * */
#include <linux/atomic.h>#include <linux/kernel.h>
#ifdef NDEBUG#define REFCOUNT_WARN(cond, str) (void)(cond)#define __refcount_check#else#define REFCOUNT_WARN(cond, str) BUG_ON(cond)#define __refcount_check __must_check#endif
typedef struct refcount_struct { atomic_t refs;} refcount_t;
#define REFCOUNT_INIT(n) { .refs = ATOMIC_INIT(n), }
static inline void refcount_set(refcount_t *r, unsigned int n){ atomic_set(&r->refs, n);}
static inline void refcount_set_release(refcount_t *r, unsigned int n){ atomic_set(&r->refs, n);}
static inline unsigned int refcount_read(const refcount_t *r){ return atomic_read(&r->refs);}
/* * Similar to atomic_inc_not_zero(), will saturate at UINT_MAX and WARN. * * Provides no memory ordering, it is assumed the caller has guaranteed the * object memory to be stable (RCU, etc.). It does provide a control dependency * and thereby orders future stores. See the comment on top. */static inline __refcount_checkbool refcount_inc_not_zero(refcount_t *r){ unsigned int old, new, val = atomic_read(&r->refs);
for (;;) { new = val + 1;
if (!val) return false;
if (unlikely(!new)) return true;
old = atomic_cmpxchg_relaxed(&r->refs, val, new); if (old == val) break;
val = old; }
REFCOUNT_WARN(new == UINT_MAX, "refcount_t: saturated; leaking memory.\n");
return true;}
/* * Similar to atomic_inc(), will saturate at UINT_MAX and WARN. * * Provides no memory ordering, it is assumed the caller already has a * reference on the object, will WARN when this is not so. */static inline void refcount_inc(refcount_t *r){ REFCOUNT_WARN(!refcount_inc_not_zero(r), "refcount_t: increment on 0; use-after-free.\n");}
/* * Similar to atomic_dec_and_test(), it will WARN on underflow and fail to * decrement when saturated at UINT_MAX. * * Provides release memory ordering, such that prior loads and stores are done * before, and provides a control dependency such that free() must come after. * See the comment on top. */static inline __refcount_checkbool refcount_sub_and_test(unsigned int i, refcount_t *r){ unsigned int old, new, val = atomic_read(&r->refs);
for (;;) { if (unlikely(val == UINT_MAX)) return false;
new = val - i; if (new > val) { REFCOUNT_WARN(new > val, "refcount_t: underflow; use-after-free.\n"); return false; }
old = atomic_cmpxchg_release(&r->refs, val, new); if (old == val) break;
val = old; }
return !new;}
static inline __refcount_checkbool refcount_dec_and_test(refcount_t *r){ return refcount_sub_and_test(1, r);}
#endif /* _ATOMIC_LINUX_REFCOUNT_H */