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2024-12-19 9370bb92b2d16684ee45cf24e879c93c509162da 
 kernel/lib/refcount.c
....@@ -1,38 +1,6 @@
11 // SPDX-License-Identifier: GPL-2.0
22 /*
3
- * 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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- *
3
+ * Out-of-line refcount functions.
364  */
375 
386 #include <linux/mutex.h>
....@@ -40,194 +8,33 @@
408 #include <linux/spinlock.h>
419 #include <linux/bug.h>
4210 
43
-/**
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- * refcount_add_not_zero_checked - 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_checked(unsigned int i, refcount_t *r)
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+#define REFCOUNT_WARN(str)	WARN_ONCE(1, "refcount_t: " str ".\n")
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+
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+void refcount_warn_saturate(refcount_t *r, enum refcount_saturation_type t)
6214 {
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-	unsigned int new, val = atomic_read(&r->refs);
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+	refcount_set(r, REFCOUNT_SATURATED);
6416 
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-	do {
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-		if (!val)
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-			return false;
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-
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-		if (unlikely(val == UINT_MAX))
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-			return true;
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-
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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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-
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-	} while (!atomic_try_cmpxchg_relaxed(&r->refs, &val, new));
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-
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-	WARN_ONCE(new == UINT_MAX, "refcount_t: saturated; leaking memory.\n");
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-
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-	return true;
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+	switch (t) {
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+	case REFCOUNT_ADD_NOT_ZERO_OVF:
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+		REFCOUNT_WARN("saturated; leaking memory");
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+		break;
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+	case REFCOUNT_ADD_OVF:
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+		REFCOUNT_WARN("saturated; leaking memory");
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+		break;
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+	case REFCOUNT_ADD_UAF:
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+		REFCOUNT_WARN("addition on 0; use-after-free");
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+		break;
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+	case REFCOUNT_SUB_UAF:
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+		REFCOUNT_WARN("underflow; use-after-free");
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+		break;
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+	case REFCOUNT_DEC_LEAK:
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+		REFCOUNT_WARN("decrement hit 0; leaking memory");
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+		break;
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+	default:
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+		REFCOUNT_WARN("unknown saturation event!?");
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+	}
8136 }
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-EXPORT_SYMBOL(refcount_add_not_zero_checked);
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-
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-/**
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- * refcount_add_checked - 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_checked(unsigned int i, refcount_t *r)
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-{
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-	WARN_ONCE(!refcount_add_not_zero_checked(i, r), "refcount_t: addition on 0; use-after-free.\n");
103
-}
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-EXPORT_SYMBOL(refcount_add_checked);
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-
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-/**
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- * refcount_inc_not_zero_checked - 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_checked(refcount_t *r)
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-{
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-	unsigned int new, val = atomic_read(&r->refs);
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-
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-	do {
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-		new = val + 1;
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-
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-		if (!val)
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-			return false;
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-
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-		if (unlikely(!new))
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-			return true;
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-
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-	} while (!atomic_try_cmpxchg_relaxed(&r->refs, &val, new));
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-
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-	WARN_ONCE(new == UINT_MAX, "refcount_t: saturated; leaking memory.\n");
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-
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-	return true;
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-}
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-EXPORT_SYMBOL(refcount_inc_not_zero_checked);
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-
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-/**
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- * refcount_inc_checked - 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_checked(refcount_t *r)
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-{
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-	WARN_ONCE(!refcount_inc_not_zero_checked(r), "refcount_t: increment on 0; use-after-free.\n");
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-}
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-EXPORT_SYMBOL(refcount_inc_checked);
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-
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-/**
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- * refcount_sub_and_test_checked - 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_checked(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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-
181
-	do {
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-		if (unlikely(val == UINT_MAX))
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-			return false;
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-
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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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-
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-	} while (!atomic_try_cmpxchg_release(&r->refs, &val, new));
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-
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-	return !new;
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-}
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-EXPORT_SYMBOL(refcount_sub_and_test_checked);
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-
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-/**
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- * refcount_dec_and_test_checked - 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_checked(refcount_t *r)
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-{
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-	return refcount_sub_and_test_checked(1, r);
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-}
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-EXPORT_SYMBOL(refcount_dec_and_test_checked);
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-
216
-/**
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- * refcount_dec_checked - 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_checked(refcount_t *r)
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-{
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-	WARN_ONCE(refcount_dec_and_test_checked(r), "refcount_t: decrement hit 0; leaking memory.\n");
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-}
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-EXPORT_SYMBOL(refcount_dec_checked);
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+EXPORT_SYMBOL(refcount_warn_saturate);
23138 
23239 /**
23340  * refcount_dec_if_one - decrement a refcount if it is 1
....@@ -269,7 +76,7 @@
26976 	unsigned int new, val = atomic_read(&r->refs);
27077 
27178 	do {
272
-		if (unlikely(val == UINT_MAX))
79
+		if (unlikely(val == REFCOUNT_SATURATED))
27380 			return true;
27481 
27582 		if (val == 1)
....@@ -294,7 +101,7 @@
294101  * @lock: the mutex to be locked
295102  *
296103  * Similar to atomic_dec_and_mutex_lock(), it will WARN on underflow and fail
297
- * to decrement when saturated at UINT_MAX.
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+ * to decrement when saturated at REFCOUNT_SATURATED.
298105  *
299106  * Provides release memory ordering, such that prior loads and stores are done
300107  * before, and provides a control dependency such that free() must come after.
....@@ -325,7 +132,7 @@
325132  * @lock: the spinlock to be locked
326133  *
327134  * Similar to atomic_dec_and_lock(), it will WARN on underflow and fail to
328
- * decrement when saturated at UINT_MAX.
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+ * decrement when saturated at REFCOUNT_SATURATED.
329136  *
330137  * Provides release memory ordering, such that prior loads and stores are done
331138  * before, and provides a control dependency such that free() must come after.