Re: [Q] Looking for an emulation for CMOV* instructions.

[David Weinehall]

On Sat, Jan 12, 2002 at 02:48:53AM -0800, Adam J. Richter wrote:
> H. Peter Anvin wrote, in response to Andi Kleen:
> >You don't need CMPXCHG8B to do efficient inline mutexes.  In fact, the
> >pthreads code for i386 uses the same mutexes the kernel does (LOCK INC
> >based, I believe), complete with section hacking to make them
> >efficiently inlinable -- and then they're put inside a function call.
> [...]
> 
> 	Your comment prompted me to look at
> linux-2.5.2-pre11/include/asm-i386/spinlock.h, and I now believe that
> the "lock; decb" that it uses for grabbing spinlocks will return an
> incorrect success if 255 or more processors are waiting on the same
> spinlock.  I don't know if anybody has ever built a shared memory x86
> multiprocessor with 257 (not a typo) or more CPU's, but it's possible
> to imagine.  It's also possible to imagine this scenario happening
> with even just one processor and a preemtable kernel.  I believe that
> the current preemtable kernel patch limits the number of preempted
> processes to something like four or six, but that's just a temporary
> limitation of the current version.

AFAIK, there's more code than this that won't work with >255
processors... It's not like each and every person on this list has
such a beast to test with (and probably most never will even if they
do exist, or come to existance...)

Regards: David Weinehall
  _                                                                 _
 // David Weinehall <[e-mail]> /> Northern lights wander      \\
//  Maintainer of the v2.0 kernel   //  Dance across the winter sky //
\>  http://www.acc.umu.se/~tao/    </   Full colour fire           </