linux-imx

// SPDX-License-Identifier: GPL-2.0

/*

 * 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().

 *

 * The decrements dec_and_test() and sub_and_test() also provide acquire

 * ordering on success.

 *

 */

​

#include <linux/mutex.h>

#include <linux/refcount.h>

#include <linux/spinlock.h>

#include <linux/bug.h>

​

/**

 * refcount_add_not_zero_checked - 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_checked(unsigned int i, refcount_t *r)

{

    unsigned int new, val = atomic_read(&r->refs);

​

    do {

        if (!val)

            return false;

​

        if (unlikely(val == UINT_MAX))

            return true;

​

        new = val + i;

        if (new < val)

            new = UINT_MAX;

​

    } while (!atomic_try_cmpxchg_relaxed(&r->refs, &val, new));

​

    WARN_ONCE(new == UINT_MAX, "refcount_t: saturated; leaking memory.\n");

​

    return true;

}

EXPORT_SYMBOL(refcount_add_not_zero_checked);

​

/**

 * refcount_add_checked - 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.