Date: Wed, 31 Jan 2001 20:18:04 -0500 From: "Matthew Emmerton" <matt@gsicomp.on.ca> To: "John Baldwin" <jhb@osd.bsdi.com> Cc: <freebsd-hackers@FreeBSD.org>, "John Baldwin" <jhb@FreeBSD.org> Subject: Re: Atomic bit operations Message-ID: <010501c08bec$d48c96a0$1200a8c0@gsicomp.on.ca> References: <XFMail.010131160650.jhb@osd.bsdi.com>
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> On 01-Feb-01 Matthew Emmerton wrote:
> >> On 31-Jan-01 Matthew Emmerton wrote:
> >> > Hi all,
> >> >
> >> > I've taken a look around for an implementation of atomic bit
operations
> > in
> >> > FreeBSD (similar to Linux' asm/bitopt.h, which include clear_bit()
and
> >> > test_and_set_bit()) but haven't found any. The only thing I've found
> > are
> >> > the atomic clear/set/add/sub routines in machine/atomic.h.
> >> >
> >> > Do we have an implementation of atomic bit operations, and if we
don't,
> >> > would we like some?
> >>
> >> Erm, atomic_set() sets's bits, and atomic_clear() clear's bits.
Anything
> > else
> >> you might need can be done with atomic_cmpset() anyways.
> >
> > Where are these functions implemented? /usr/include/machine/atomic.h
just
> > has atomic_set_XXX and clear_XXX primitives, which work on
char/short/longs,
> > but not individual bits.
>
> 1) You need to look in -current.
I only recently moved to 4-stable, so I'm not going to jump to -current
quite yet :)
> 2) atomic_set_int(&my_int, 4); sets bit _2_ in the integer variable
my_int.
> Make sense? You can't address individual bits on a machine. :-P
>
> > I presume that I could wrap the char operations with something that
takes
> > 0x01 and bit-shifts it appropriately so that atomic_set/clear could be
used.
>
> Hmm, so you mean the API is atomic_set(&foo, x) implies foo |= 1 << x?
> That means you can't set more than 1 bit atomically, which is a bit
> limiting.
Yes, but it's the API used by some code I recently inherited. (Recall that
this API is Linux's asm/bitops.h) I realize that the existing
atomic_xxx_xxx functions are much more flexible than bit-based ones, but
wouldn't it make sense to have the full complement of functions available?
> > However, certain other primitives are missing, such as an atomic
> > test-and-set operation. Under the current scheme this would have to be
done
> > by two independent operations, which is not useful when atomicitiy is
> > required.
>
> do {
> x = my_int;
> while (atomic_cmpset_int(&my_int, x, x | foo) == 0);
> if (x & foo)
> foo_was_already_set;
> else
> foo_was_not_already_set;
I had based my assumptions on what I saw in 4-stable, which doesn't have
atomic_cmpset. I'll have to take a peek at 4-current or wait until some
features trickle down to 4-stable.
Thanks,
--
Matt Emmerton
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