atomic_add, atomic_sub, atomic_inc, atomic_dec -
       thread/SMP safe arithmetic on atomic data

SYNOPSIS

       #include <asm/atomic.h>

       void atomic_add(int i, volatile atomic_t*v)
       void atomic_sub(int i, volatile atomic_t*v)
       void atomic_inc(volatile atomic_t*v)
       void atomic_dec(volatile atomic_t*v)
       int atomic_read(volatile atomic_t*v)
       void atomic_set(volatile atomic_t*v, int i)
       int atomic_dec_and_test(volatile atomic_t*v)

DESCRIPTION

       These functions manipulate variables of type  atomic_t  is
       SMP  and  interrupt safe ways. These variables can be used
       to hold spin locks or SMP-safe reference  counters.  These
       functions guarantee that the operation that they represent
       is performed correctly. If necessary, hardware bus locking
       is  performed  to protect the operation.  Usually, the CPU
       has some sort of  atomic  instructions  that  allow  these
       operations to be performed quickly and safely.

       The  atomic_dec_and_test  decrements  the atomic variable,
       and returns true if the result is zero. This  function  is
       particularly useful in implementing spin locks on SMP sys­
       tems.

RETURN VALUE

       The atomic_read function returns the integer value of  the
       atomic variable.

       The  atomic_dec_and_test returns TRUE if the value becomes
       zero after the decrement.

AVAILABILITY

       Linux 2.0+

SEE ALSO

       intro(9)

AUTHOR

       Stephen Williams <steve@icarus.com>

BUGS

       The read and set operations generally have no special pro­
       tections.