Certain operations are guaranteed atomic on all platforms. The
first class of operations work on atomic_t
`include/asm/atomic.h`; this
contains a signed integer (at least 24 bits long), and you must use
these functions to manipulate or read atomic_t variables.
`atomic_read()` and
`atomic_set()` get and set the counter,
`atomic_add()`,
`atomic_sub()`,
`atomic_inc()`,
`atomic_dec()`, and
`atomic_dec_and_test()` (returns
true if it was decremented to zero).

Yes. It returns true (i.e. != 0) if the atomic variable is zero.

Note that these functions are slower than normal arithmetic, and so should not be used unnecessarily. On some platforms they are much slower, like 32-bit Sparc where they use a spinlock.

The second class of atomic operations is atomic bit operations on a
long, defined in
`include/asm/bitops.h`. These
operations generally take a pointer to the bit pattern, and a bit
number: 0 is the least significant bit.
`set_bit()`, `clear_bit()`
and `change_bit()` set, clear, and flip the
given bit. `test_and_set_bit()`,
`test_and_clear_bit()` and
`test_and_change_bit()` do the same thing,
except return true if the bit was previously set; these are
particularly useful for very simple locking.

It is possible to call these operations with bit indices greater than BITS_PER_LONG. The resulting behavior is strange on big-endian platforms though so it is a good idea not to do this.

Note that the order of bits depends on the architecture, and in particular, the bitfield passed to these operations must be at least as large as a long.