[9fans] _xinc vs ainc

[erik quanstrom <quanstro@quanstro.net>]

i'm confused by the recent change to the thread library.
the old code was simply to do a locked incl.  the new code
does a locked exchange /within a loop/ until it's seen that
nobody else has updated the value at the same time, thus
insuring that the value has indeed been updated.

since the expensive operation is the MESI(F) negotiation
behind the scenes to get exclusive access to the cacheline,
i don't understand the motiviation is for replacing _xinc
with ainc.  since ainc can loop on an expensive lock instruction.

that is, i think the old version was wait free, and the new version
is not.

can someone explain what i'm missing here?

thanks!

- erik

----

TEXT	_xinc(SB),$0	/* void _xinc(long *); */

	MOVL	l+0(FP),AX
	LOCK
	INCL	0(AX)
	RET

----

TEXT ainc(SB), $0	/* long ainc(long *); */
	MOVL	addr+0(FP), BX
ainclp:
	MOVL	(BX), AX
	MOVL	AX, CX
	INCL	CX
	LOCK
	BYTE	$0x0F; BYTE $0xB1; BYTE $0x0B	/* CMPXCHGL CX, (BX) */
	JNZ	ainclp
	MOVL	CX, AX
	RET