cmpxchg / atomic_cmpset_int emulation for userland (i386) ?

[Kostik Belousov]

On Thu, Apr 02, 2009 at 10:58:54AM +0200, Luigi Rizzo wrote:
> 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.

Support for FPU-less operations was removed from HEAD in Jul 2003.
The support for i386+387 seems to be removed in 2004, i.e. before 5.x.
The kernel explicitely requires working read-only mappings of the
pages for kernel mode, AKA WP bit in CR0. Also, kernel assumes that
cmpxchgl is always present.

Do you want to support 4.x ?
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