Re: [PATCH] Add FUTEX_CMP_REQUEUE futex op

[Jakub Jelinek <jakub@redhat.com>]

On Thu, May 20, 2004 at 11:36:39PM -0700, Andrew Morton wrote:
> > > It'll work OK on x86 because of the stack layout but is the same true of
> > > all other supported architectures?
> > 
> > We add parameters at the end.  This does not influence how previous
> > values are passed.  And especially for syscalls it makes no difference.
> > 
> 
> what we're effectively doing is:
> 
> void foo(int a, int b, int c)
> {
> }
> 
> and from another compilation unit:
> 
> 	foo(a, b);
> 
> and we're expecting the a's and b's to line up across all architectures and
> compiler options.  I thought that on some architectures that only works out
> if the function has a vararg declaration.

The kernel syscall ABI is on many arches different from the compiler ABI.
Adding an argument this way certainly works on the architectures I'm
familiar with (i386, x86_64, ia64, ppc, ppc64, s390, s390x, sparc, sparc64,
alpha).  I believe arm will work too, don't keep track of the rest of
arches.

Well, for s390/s390x there is a problem that it doesn't allow (yet) 6
argument syscalls at all, so one possibility for s390* is adding a wrapper around
sys_futex which will take the 5th and 6th arguments for FUTEX_CMP_REQUEUE
from a structure pointed to by 5th argument and pass that to sys_futex.

If some weirdo arch has problems with this, it can certainly deal with it in
its architecture wrapper.

	Jakub