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UCAS(9)                NetBSD Kernel Developer's Manual                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 consid-
     ered 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 pro-
     vide 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 suit-
     able 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 synchro-
     nizes 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 con-
     sult the value returned via retp to check the result of the CAS opera-
     tion.  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.

NetBSD 10.99                    March 31, 2019                    NetBSD 10.99