RE: [PATCH] udev enhancements to use kernel event queue

[Davide Libenzi]

On Fri, 13 Jun 2003, David Schwartz wrote:

> 	You could also do (in pseudo-code):
>
> top:
>  ret <- v->counter
>  inc ret
>  LOCK incl v->counter
>  cmp v->counter, ret
>  jz end
>  LOCK decl v->counter
>  jmp top:
> end:
>  return ret
>
> 	This does not strictly guarantee in order return values, but that's
> meaningless without a lock anyway.

It is even worse since it can return same values. Suppose counter is 0 :

TASK0                   TASK1

top:
ret <- v->counter     ; ret = 0
inc ret               ; ret -> 1
LOCK incl v->counter  ; counter -> 1

			top:
			ret <- v->counter     ; ret = 1
			inc ret               ; ret -> 2
			LOCK incl v->counter  ; counter -> 2
			cmp v->counter, ret   ; match !! got 2
			jz end
			...

cmp v->counter, ret  ; ret == 1 , counter == 2 no-match
jz end
LOCK decl v->counter ; counter -> 1
jmp top

Then TASK0 starts again with no interruption and gets 2. You need
instrucions that does atomic mod/cmp to do this kind of tricks.

- Davide