NAME

     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 per-
     formed correctly. If necessary, hardware bus locking is per-
     formed  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 partic-
     ularly useful in implementing spin locks on SMP systems.

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.