353 lines
10 KiB
C
353 lines
10 KiB
C
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// SPDX-License-Identifier: GPL-2.0
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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/refcount.h>
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#include <linux/bug.h>
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#ifdef CONFIG_REFCOUNT_FULL
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/**
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* refcount_add_not_zero - add a value to a refcount unless it is 0
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* @i: the value to add to the refcount
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* @r: the refcount
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*
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* 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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* Use of this function is not recommended for the normal reference counting
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* use case in which references are taken and released one at a time. In these
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* cases, refcount_inc(), or one of its variants, should instead be used to
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* increment a reference count.
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*
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* Return: false if the passed refcount is 0, true otherwise
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*/
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bool refcount_add_not_zero(unsigned int i, refcount_t *r)
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{
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unsigned int new, val = atomic_read(&r->refs);
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do {
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if (!val)
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return false;
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if (unlikely(val == UINT_MAX))
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return true;
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new = val + i;
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if (new < val)
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new = UINT_MAX;
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} while (!atomic_try_cmpxchg_relaxed(&r->refs, &val, new));
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WARN_ONCE(new == UINT_MAX, "refcount_t: saturated; leaking memory.\n");
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return true;
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}
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EXPORT_SYMBOL(refcount_add_not_zero);
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/**
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* refcount_add - add a value to a refcount
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* @i: the value to add to the refcount
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* @r: the refcount
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*
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* Similar to atomic_add(), but 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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* Use of this function is not recommended for the normal reference counting
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* use case in which references are taken and released one at a time. In these
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* cases, refcount_inc(), or one of its variants, should instead be used to
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* increment a reference count.
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*/
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void refcount_add(unsigned int i, refcount_t *r)
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{
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WARN_ONCE(!refcount_add_not_zero(i, r), "refcount_t: addition on 0; use-after-free.\n");
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}
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EXPORT_SYMBOL(refcount_add);
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/**
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* refcount_inc_not_zero - increment a refcount unless it is 0
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* @r: the refcount to increment
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*
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* Similar to atomic_inc_not_zero(), but 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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* Return: true if the increment was successful, false otherwise
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*/
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bool refcount_inc_not_zero(refcount_t *r)
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{
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unsigned int new, val = atomic_read(&r->refs);
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do {
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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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} while (!atomic_try_cmpxchg_relaxed(&r->refs, &val, new));
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WARN_ONCE(new == UINT_MAX, "refcount_t: saturated; leaking memory.\n");
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return true;
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}
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EXPORT_SYMBOL(refcount_inc_not_zero);
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/**
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* refcount_inc - increment a refcount
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* @r: the refcount to increment
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*
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* Similar to atomic_inc(), but 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.
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*
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* Will WARN if the refcount is 0, as this represents a possible use-after-free
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* condition.
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*/
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void refcount_inc(refcount_t *r)
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{
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WARN_ONCE(!refcount_inc_not_zero(r), "refcount_t: increment on 0; use-after-free.\n");
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}
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EXPORT_SYMBOL(refcount_inc);
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/**
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* refcount_sub_and_test - subtract from a refcount and test if it is 0
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* @i: amount to subtract from the refcount
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* @r: the refcount
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*
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* Similar to atomic_dec_and_test(), but it will WARN, return false and
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* ultimately leak on underflow and will fail to decrement when saturated
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* 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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* Use of this function is not recommended for the normal reference counting
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* use case in which references are taken and released one at a time. In these
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* cases, refcount_dec(), or one of its variants, should instead be used to
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* decrement a reference count.
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*
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* Return: true if the resulting refcount is 0, false otherwise
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*/
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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 new, val = atomic_read(&r->refs);
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do {
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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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WARN_ONCE(new > val, "refcount_t: underflow; use-after-free.\n");
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return false;
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}
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} while (!atomic_try_cmpxchg_release(&r->refs, &val, new));
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return !new;
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}
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EXPORT_SYMBOL(refcount_sub_and_test);
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/**
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* refcount_dec_and_test - decrement a refcount and test if it is 0
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* @r: the refcount
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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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* Return: true if the resulting refcount is 0, false otherwise
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*/
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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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EXPORT_SYMBOL(refcount_dec_and_test);
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/**
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* refcount_dec - decrement a refcount
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* @r: the refcount
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*
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* Similar to atomic_dec(), it will WARN on underflow and fail to decrement
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* 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.
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*/
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void refcount_dec(refcount_t *r)
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{
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WARN_ONCE(refcount_dec_and_test(r), "refcount_t: decrement hit 0; leaking memory.\n");
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}
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EXPORT_SYMBOL(refcount_dec);
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#endif /* CONFIG_REFCOUNT_FULL */
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/**
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* refcount_dec_if_one - decrement a refcount if it is 1
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* @r: the refcount
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*
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* No atomic_t counterpart, it attempts a 1 -> 0 transition and returns the
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* success thereof.
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*
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* Like all decrement operations, it provides release memory order and provides
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* a control dependency.
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*
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* It can be used like a try-delete operator; this explicit case is provided
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* and not cmpxchg in generic, because that would allow implementing unsafe
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* operations.
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*
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* Return: true if the resulting refcount is 0, false otherwise
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*/
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bool refcount_dec_if_one(refcount_t *r)
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{
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int val = 1;
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return atomic_try_cmpxchg_release(&r->refs, &val, 0);
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}
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EXPORT_SYMBOL(refcount_dec_if_one);
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/**
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* refcount_dec_not_one - decrement a refcount if it is not 1
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* @r: the refcount
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*
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* No atomic_t counterpart, it decrements unless the value is 1, in which case
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* it will return false.
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*
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* Was often done like: atomic_add_unless(&var, -1, 1)
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*
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* Return: true if the decrement operation was successful, false otherwise
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*/
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bool refcount_dec_not_one(refcount_t *r)
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{
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unsigned int new, val = atomic_read(&r->refs);
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do {
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if (unlikely(val == UINT_MAX))
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return true;
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if (val == 1)
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return false;
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new = val - 1;
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if (new > val) {
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WARN_ONCE(new > val, "refcount_t: underflow; use-after-free.\n");
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return true;
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}
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} while (!atomic_try_cmpxchg_release(&r->refs, &val, new));
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return true;
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}
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EXPORT_SYMBOL(refcount_dec_not_one);
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/**
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* refcount_dec_and_mutex_lock - return holding mutex if able to decrement
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* refcount to 0
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* @r: the refcount
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* @lock: the mutex to be locked
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*
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* Similar to atomic_dec_and_mutex_lock(), it will WARN on underflow and fail
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* to 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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* Return: true and hold mutex if able to decrement refcount to 0, false
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* otherwise
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*/
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bool refcount_dec_and_mutex_lock(refcount_t *r, struct mutex *lock)
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{
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if (refcount_dec_not_one(r))
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return false;
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mutex_lock(lock);
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if (!refcount_dec_and_test(r)) {
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mutex_unlock(lock);
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return false;
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}
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return true;
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}
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EXPORT_SYMBOL(refcount_dec_and_mutex_lock);
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/**
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* refcount_dec_and_lock - return holding spinlock if able to decrement
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* refcount to 0
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* @r: the refcount
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* @lock: the spinlock to be locked
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*
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* Similar to atomic_dec_and_lock(), 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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* Return: true and hold spinlock if able to decrement refcount to 0, false
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* otherwise
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*/
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bool refcount_dec_and_lock(refcount_t *r, spinlock_t *lock)
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{
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if (refcount_dec_not_one(r))
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return false;
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spin_lock(lock);
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if (!refcount_dec_and_test(r)) {
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spin_unlock(lock);
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return false;
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}
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return true;
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}
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EXPORT_SYMBOL(refcount_dec_and_lock);
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