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SubjectRe: FD array expansion problem
On Thu, Oct 28, 1999 at 01:35:19PM +0200, Matthew Wilcox wrote:

> > BTW, does anyone know why fd array manipulations are performed via atomic
> > operation (xchg) in some places (fd_install, expand_fdset)?
> > It looks strange because
> > 1) atomic operations don't cover all accesses;
> > 2) these code parts are already protected by a write lock.
>
> I'd like to just plead with people _not_ to use xchg unnecessarily.
> On PA-RISC, it is going to _suck_. PA-RISC's only atomic instruction
> is load-and-zero. Therefore we have to have a spinlock in our generic
> xchg implementation. And disable interrupts. Ugh. It's OK if you
> xchg something with zero though, and most of the xchg's in the kernel
> are with zero.

MIPS II instruction set and better and the Alpha don't have an xchg
equivalent but ll / sc instructions, which are more universal. Using
them to implement an xchg-like implementation however ends up as a
a loop of at least five instructions which we'de prefarably want to avoid.

The MIPS I (R2000, R3000, embedded toys) situation is worse, they don't
have atomic operations at all. Historical side note - external hardware
was being used to implement atomic operations on such systems for SMP. I
don't know the details but what I know looks bad enough that I probably
will refuse to add R3000 SMP support. Performance is even worse, I've
been told that atomic implementations were taking a number of microseconds
in the range of three to four digits. Imagine, *miliseconds*.

Currently we have two types of atomic operations in the kernel, atomic
with respect to interrupts including bottom half handlers and atomic with
respect to other processors in a MP system. We however only have a
single implementation of all atomic operations.

Ralf

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