Subject: Re: newlock
To: YAMAMOTO Takashi <firstname.lastname@example.org>
From: Andrew Doran <ad@NetBSD.org>
Date: 09/04/2006 02:51:33
On Mon, Sep 04, 2006 at 08:47:38AM +0900, YAMAMOTO Takashi wrote:
> > The assumptions were that we would be dealing with two or more locks, with the
> > same SPL, and that the acquire and release would be done locally. That's not
> > general enough though. So, following your suggestion, something like this:
> > mutex_enter(a);
> > .. find b ..
> > mutex_enter(b);
> > mutex_link(b, mutex_exit_linked(a));
> > .. update b ..
> > mutex_exit(b);
> > This is a part of the design I'm not comfortable with, and I think the above
> > is quite ugly. What do people think?
> what's wrong with using plain old splxxx() where out-of-order operation is
> really necessary? i don't think it's often.
David's suggestion has a couple of advantages that I can see:
- You can raise the SPL via mutex_enter() in find_foo_and_return_locked(),
and not have to worry about raising or restoring it around the call to the
function. The mutex contains the saved SPL.
- The SPL is specified in one place.
I guess the the question is whether it's necessary. With the way the mutexes
are imlemented, it's certianly cheap to do.