Date: Thu, 2 Apr 2009 10:58:54 +0200 From: Luigi Rizzo <rizzo@iet.unipi.it> To: Kostik Belousov <kostikbel@gmail.com> Cc: current@freebsd.org Subject: Re: cmpxchg / atomic_cmpset_int emulation for userland (i386) ? Message-ID: <20090402085854.GA2237@onelab2.iet.unipi.it> In-Reply-To: <20090402084833.GZ31897@deviant.kiev.zoral.com.ua> References: <20090402070605.GA96848@onelab2.iet.unipi.it> <20090402084833.GZ31897@deviant.kiev.zoral.com.ua>
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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
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