Subject: Re: Test and set (was Re: postgreSQL)
To: Chris G. Demetriou <firstname.lastname@example.org>
From: Simon Burge <email@example.com>
Date: 05/26/1999 23:01:04
Chris G. Demetriou wrote:
> Simon Burge <firstname.lastname@example.org> writes:
> > NetBSD doesn't have anything like this - would it be a waste of a system
> > call (or a possibly system-call backed library routine) to add some sort
> > of MI atomic test and set operation?
> Yes, it would be a waste of a syscall.
> This is best implemented without kernel interaction if possible,
> because of the cost of of a syscall. that's possible on several
> On architectures where it has to be implemented as a sycall, the right
> syscall already exists (sysarch()).
> A standard API would be fine, good actually.
As far as I can tell, this is a DEC API, not a MIPS API. But it is most
useful on MIPS, so I guess the "right" thing to do is implement it as an
architecture-specific call and not a machine specific call. Would we
need to add the same API for VAX/Ultrix and Alpha/OSF1 compatibility as
well? And would it go in libc or libcompat?
> > The R4000 and later MIPS processors (actually MIPS II CPUs and later)
> > have the LL (Load Linked) and SC (Store Conditional) instructions, but
> > this really doesn't help everyone :-( Are there any other archs other
> > than MIPS that don't have a test and set available to user code?
> really, on the mips, what you probably want is a library
> routine/function pointer which on first run checks the cpu type and
> replaces the function pointer either with one that invokes a the LL/SC
> instructions or invokes the syscall.
This is what I meant by "or a possibly system-call backed library
routine". From what I can tell, on the MIPS you can't tell which CPU
type you have from userland so we'd need a sysctl to give us the CPU
architecture. It looks as though we don't have the machinery in place
(on MIPS at least) to support arch-dependant vs. cpu-dependant sysctl's
at the moment - I'll look into that.
So we'd end up with something like for MIPS:
atomic_op(foo, bar, ...)
static int cpu_arch = 0;
if (cpu_arch == 0)
if (cpu_arch == 1)
/* No LL/SC on MIPS I */