Commit bd174169 authored by David Windsor's avatar David Windsor Committed by Ingo Molnar

locking/refcount: Add refcount_t API kernel-doc comments

Signed-off-by: default avatarDavid Windsor <dwindsor@gmail.com>
Acked-by: default avatarPeter Zijlstra <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: elena.reshetova@intel.com
Cc: kernel-hardening@lists.openwall.com
Link: http://lkml.kernel.org/r/1489160052-20293-1-git-send-email-dwindsor@gmail.comSigned-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent 4495c08e
......@@ -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);
......
......@@ -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);
......@@ -64,18 +82,39 @@ 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)
{
......@@ -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)
{
......
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