refcount: add refcount_t API kernel-doc comments

v3: rebase against latest Linus tree; re-add include/linux/refcount.h missing from v2 series

Signed-off-by David Windsor <[email protected]>

---
 include/linux/refcount.h |  19 ++++++++
 lib/refcount.c           | 124 ++++++++++++++++++++++++++++++++++++++++++-----
 2 files changed, 130 insertions(+), 13 deletions(-)

include/linux/refcount.h +19 -0

index 0023fee..b34aa64 100644
@@ -6,17 +6,36 @@
 #include <linux/spinlock.h>
 #include <linux/kernel.h>
 
+/**
+ * refcount_t - variant of atomic_t specialized for reference counts
+ * @refs: atomic_t counter field
+ *
+ * The counter saturates at UINT_MAX and will not move once
+ * there. This avoids wrapping the counter and causing 'spurious'
+ * use-after-free bugs.
+ */
 typedef struct refcount_struct {
        atomic_t refs;
 } refcount_t;
 
 #define REFCOUNT_INIT(n)       { .refs = ATOMIC_INIT(n), }
 
+/**
+ * refcount_set - set a refcount's value
+ * @r: the refcount
+ * @n: value to which the refcount will be set
+ */
 static inline void refcount_set(refcount_t *r, unsigned int n)
 {
        atomic_set(&r->refs, n);
 }
 
+/**
+ * refcount_read - get a refcount's value
+ * @r: the refcount
+ *
+ * Return: the refcount's value
+ */
 static inline unsigned int refcount_read(const refcount_t *r)
 {
        return atomic_read(&r->refs);

lib/refcount.c +111 -14

index aa09ad3..8d5d680 100644
@@ -37,6 +37,24 @@
 #include <linux/refcount.h>
 #include <linux/bug.h>
 
+/**
+ * refcount_add_not_zero - add a value to a refcount unless it is 0
+ * @i: the value to add to the refcount
+ * @r: the refcount
+ *
+ * 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.
+ *
+ * Use of this function is not recommended for the normal reference counting
+ * use case in which references are taken and released one at a time.  In these
+ * cases, refcount_inc(), or one of its variants, should instead be used to
+ * increment a reference count.
+ *
+ * Return: false if the passed refcount is 0, true otherwise
+ */
 bool refcount_add_not_zero(unsigned int i, refcount_t *r)
 {
        unsigned int old, new, val = atomic_read(&r->refs);

@@ -64,19 +82,40 @@ bool refcount_add_not_zero(unsigned int i, refcount_t *r)
 }
 EXPORT_SYMBOL_GPL(refcount_add_not_zero);
 
+/**
+ * refcount_add - add a value to a refcount
+ * @i: the value to add to the refcount
+ * @r: the refcount
+ *
+ * Similar to atomic_add(), but 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.
+ *
+ * Use of this function is not recommended for the normal reference counting
+ * use case in which references are taken and released one at a time.  In these
+ * cases, refcount_inc(), or one of its variants, should instead be used to
+ * increment a reference count.
+ */
 void refcount_add(unsigned int i, refcount_t *r)
 {
        WARN_ONCE(!refcount_add_not_zero(i, r), "refcount_t: addition on 0; use-after-free.\n");
 }
 EXPORT_SYMBOL_GPL(refcount_add);
 
-/*
- * Similar to atomic_inc_not_zero(), will saturate at UINT_MAX and WARN.
+/**
+ * refcount_inc_not_zero - increment a refcount unless it is 0
+ * @r: the refcount to increment
+ *
+ * Similar to atomic_inc_not_zero(), but 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.
- */
+ *
+ * Return: true if the increment was successful, false otherwise
+ */
 bool refcount_inc_not_zero(refcount_t *r)
 {
        unsigned int old, new, val = atomic_read(&r->refs);

@@ -103,11 +142,17 @@ bool refcount_inc_not_zero(refcount_t *r)
 }
 EXPORT_SYMBOL_GPL(refcount_inc_not_zero);
 
-/*
- * Similar to atomic_inc(), will saturate at UINT_MAX and WARN.
+/**
+ * refcount_inc - increment a refcount
+ * @r: the refcount to increment
+ *
+ * Similar to atomic_inc(), but 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.
+ * reference on the object.
+ *
+ * Will WARN if the refcount is 0, as this represents a possible use-after-free
+ * condition.
  */
 void refcount_inc(refcount_t *r)
 {

@@ -115,6 +160,26 @@ void refcount_inc(refcount_t *r)
 }
 EXPORT_SYMBOL_GPL(refcount_inc);
 
+/**
+ * refcount_sub_and_test - subtract from a refcount and test if it is 0
+ * @i: amount to subtract from the refcount
+ * @r: the refcount
+ *
+ * Similar to atomic_dec_and_test(), but it will WARN, return false and
+ * ultimately leak on underflow and will 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.
+ *
+ * Use of this function is not recommended for the normal reference counting
+ * use case in which references are taken and released one at a time.  In these
+ * cases, refcount_dec(), or one of its variants, should instead be used to
+ * decrement a reference count.
+ *
+ * Return: true if the resulting refcount is 0, false otherwise
+ */
 bool refcount_sub_and_test(unsigned int i, refcount_t *r)
 {
        unsigned int old, new, val = atomic_read(&r->refs);

@@ -140,13 +205,18 @@ bool refcount_sub_and_test(unsigned int i, refcount_t *r)
 }
 EXPORT_SYMBOL_GPL(refcount_sub_and_test);
 
-/*
+/**
+ * refcount_dec_and_test - decrement a refcount and test if it is 0
+ * @r: the refcount
+ *
  * 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.
+ *
+ * Return: true if the resulting refcount is 0, false otherwise
  */
 bool refcount_dec_and_test(refcount_t *r)
 {

@@ -154,21 +224,26 @@ bool refcount_dec_and_test(refcount_t *r)
 }
 EXPORT_SYMBOL_GPL(refcount_dec_and_test);
 
-/*
+/**
+ * refcount_dec - decrement a refcount
+ * @r: the refcount
+ *
  * Similar to atomic_dec(), 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.
  */
-
 void refcount_dec(refcount_t *r)
 {
        WARN_ONCE(refcount_dec_and_test(r), "refcount_t: decrement hit 0; leaking memory.\n");
 }
 EXPORT_SYMBOL_GPL(refcount_dec);
 
-/*
+/**
+ * refcount_dec_if_one - decrement a refcount if it is 1
+ * @r: the refcount
+ *
  * No atomic_t counterpart, it attempts a 1 -> 0 transition and returns the
  * success thereof.
  *

@@ -178,6 +253,8 @@ EXPORT_SYMBOL_GPL(refcount_dec);
  * It can be used like a try-delete operator; this explicit case is provided
  * and not cmpxchg in generic, because that would allow implementing unsafe
  * operations.
+ *
+ * Return: true if the resulting refcount is 0, false otherwise
  */
 bool refcount_dec_if_one(refcount_t *r)
 {

@@ -185,11 +262,16 @@ bool refcount_dec_if_one(refcount_t *r)
 }
 EXPORT_SYMBOL_GPL(refcount_dec_if_one);
 
-/*
+/**
+ * refcount_dec_not_one - decrement a refcount if it is not 1
+ * @r: the refcount
+ *
  * No atomic_t counterpart, it decrements unless the value is 1, in which case
  * it will return false.
  *
  * Was often done like: atomic_add_unless(&var, -1, 1)
+ *
+ * Return: true if the decrement operation was successful, false otherwise
  */
 bool refcount_dec_not_one(refcount_t *r)
 {

@@ -219,13 +301,21 @@ bool refcount_dec_not_one(refcount_t *r)
 }
 EXPORT_SYMBOL_GPL(refcount_dec_not_one);
 
-/*
+/**
+ * refcount_dec_and_mutex_lock - return holding mutex if able to decrement
+ *                               refcount to 0
+ * @r: the refcount
+ * @lock: the mutex to be locked
+ *
  * Similar to atomic_dec_and_mutex_lock(), 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.
+ *
+ * Return: true and hold mutex if able to decrement refcount to 0, false
+ *         otherwise
  */
 bool refcount_dec_and_mutex_lock(refcount_t *r, struct mutex *lock)
 {

@@ -242,13 +332,21 @@ bool refcount_dec_and_mutex_lock(refcount_t *r, struct mutex *lock)
 }
 EXPORT_SYMBOL_GPL(refcount_dec_and_mutex_lock);
 
-/*
+/**
+ * refcount_dec_and_lock - return holding spinlock if able to decrement
+ *                         refcount to 0
+ * @r: the refcount
+ * @lock: the spinlock to be locked
+ *
  * Similar to atomic_dec_and_lock(), 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.
+ *
+ * Return: true and hold spinlock if able to decrement refcount to 0, false
+ *         otherwise
  */
 bool refcount_dec_and_lock(refcount_t *r, spinlock_t *lock)
 {
--

2.7.4

Quoted 4 lines · David Windsor

v3: rebase against latest Linus tree; re-add include/linux/refcount.h missing from v2 series

Signed-off-by David Windsor <[email protected]>

Reviewed-by Kees Cook <[email protected]>

Ingo, can you pull this into tip? I like having more documentation here. :)

Quoted 9 lines · Kees Cook

Quoted 4 lines · David Windsor

v3: rebase against latest Linus tree; re-add include/linux/refcount.h missing from v2 series

Signed-off-by David Windsor <[email protected]>

Reviewed-by Kees Cook <[email protected]>

Ingo, can you pull this into tip? I like having more documentation here. :)

So I hopped into my time machine and applied it to tip:locking/core as of March 13th:

commit bd174169c7a12a37b3b4aa2221f084ade010b182
Author: David Windsor <[email protected]>
AuthorDate: Fri Mar 10 10:34:12 2017 -0500
Commit: Ingo Molnar <[email protected]>
CommitDate: Mon Mar 13 07:41:08 2017 +0100

You are welcome! ;-)