UCAS(9)

NAME
ucas - atomic memory operations on user-space address

SYNOPSIS
#include <sys/systm.h>
int
ucas_ptr(volatile void *uptr, void *old, void *new, void *retp);
int
ucas_int(volatile int *uptr, int old, int new, int *retp);

DESCRIPTION
These functions provide compare-and-swap (CAS) functionality on user-space address.

Except that they can be safely used for the kernel to access user-space address, they are semantically equivalents of atomic_cas(3).

uptr
    The pointer to the variable. This should be a user-space pointer.

old The value to compare with the variable.

new The value to store to the variable.

retp
    The pointer to the memory to store the old value of the variable.

IMPLEMENTATION NOTES
The ucas functions are implemented in machine-independent code, but rely on machine-dependent code to implement optimized primitives, if possible.

The basic ucas primitives have the following signatures and are considered private to the implementation and are not to be called by consumers of the ucas API:

int _ucas_32(volatile uint32_t *uptr, uint32_t old, uint32_t new, uint32_t *retp);

int _ucas_64(volatile uint64_t *uptr, uint64_t old, uint64_t new, uint64_t *retp);

If a platform is able to provide a CAS operation that meets the following criteria, it should define __HAVE_UCAS_FULL in <machine/types.h> and provide complete machine-dependent implementations of _ucas_32() ( and _ucas_64(), if an _LP64 platform):

-   Can be implemented using either native compare-and-swap operations or load-locked / store-conditional code sequences.
-   Can be used on uniprocessor and multiprocessor systems.
-   Can operate across the kernel-userspace boundary.

If __HAVE_UCAS_FULL is not defined, then a generic implementation will be provided by machine-dependent code. This generic implementation is suitable for uniprocessor and multiprocessor systems, but works on a “least-common denominator” principle. In particular, kernel preemption is disabled during the critical section ( which is comprised of ufetch(9) and ustore(9) operations), and the multiprocessor implementation synchronizes with other CPUs using interprocessor interrupts.

If a particular platform wishes to use the generic implementation on uniprocessors but an optimized implementation on multiprocessors, the platform should define __HAVE_UCAS_MP in <machine/types.h> and provide _ucas_32_mp() ( and _ucas_64_mp(), if an _LP64 platform).

RETURN VALUES
On success, these functions return 0. In that case, the caller can consult the value returned via retp to check the result of the CAS operation. Otherwise, these functions return an appropriate errno(9) error code, typically EFAULT.

SEE ALSO
atomic_cas(3), intro(9)

BUGS
Conceptually, the retp argument of ucas_ptr() would be of void **. The current prototype is a compromise for usability.