cmpxchg / atomic_cmpset_int emulation for userland (i386) ?

[Kostik Belousov]

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.
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