Re: implementing Futex

[Michael Schnell <mschnell@lumino.de>]

Arnd Bergmann wrote:,
>> From reading the code (futex_atomic_op_inuser() seems only to be called
>> once (in futex.c) ), it seems that futex_atomic_op_inuser() is not
>> really used at all. It seems that it'd only called for futex_wake, and
>> here the "Operation" is derived from the last parameter of the system
>> call, which the man page says is ignored.  So, are all the operations
>> implemented there really necessary or just "nice to have" ?
>
> You misread futex_wake_op(). The operation comes from the second parameter
> of the syscall, not the last one.

Really ?

In futex.c, I see

static int
futex_wake_op(u32 __user *uaddr1, int fshared, u32 __user *uaddr2,
	      int nr_wake, int nr_wake2, int op) {
  ..
 retry_private:
	op_ret = futex_atomic_op_inuser(op, uaddr2);
  ..
}
...
long do_futex(u32 __user *uaddr, int op, u32 val, ktime_t *timeout,
		u32 __user *uaddr2, u32 val2, u32 val3) {	
  ..
  case   FUTEX_WAKE_OP:
		ret = futex_wake_op(uaddr, fshared, uaddr2,
                      val, val2, val3);
  ..
};
...
SYSCALL_DEFINE6(futex, u32 __user *, uaddr, int, op, u32, val,
		struct timespec __user *, utime, u32 __user *, uaddr2,
		u32, val3) {
  ..	
  return do_futex(uaddr, op, val, tp, uaddr2, val2, val3);
  ..
}

I don't see that the value in question is modified anywhere in that code.

What am I getting wrong ?

But I also don't see how this makes any sense.

-Michael