cmpxchg / atomic_cmpset_int emulation for userland (i386) ?

[Luigi Rizzo]

On Thu, Apr 02, 2009 at 11:48:33AM +0300, Kostik Belousov wrote:
> On Thu, Apr 02, 2009 at 09:06:05AM +0200, Luigi Rizzo wrote:
> > Hi,
> > I have some list manipulation algorithm that I would like to use
> > that relies rather centrally on atomic_cmpset_int().
> > 
> > This is an atomic instruction on 486+, but not available on 386
> > and maybe other platforms. i386/atomic.h has a replacement
> > but it uses "pushfl; cli; ... popfl;" so it cannot run in userland.
> > 
> > I was wondering if there is a good emulation for that instruction
> > on the i386 that is suitable for userland (other architectures
> > we support have a CPU instruction that does it, or in the case of ARM,
> > a usable emulation for userland).
> 
> FreeBSD cannot boot on anything < 486, i.e. cmpxchgl and xaddl may be
> considered always supported by the CPU.

It was a slightly more generic question -- this stuff is for userland
so while we can assume it works on modern FreeBSD versions, I would
like to see what constraints it has on older versions of FreeBSD.
Of course I can emulate the critical section with a pthread lock,
but that would be the worst case option.

	cheers
	luigi