From owner-freebsd-hackers  Wed Jan 31 16: 7:48 2001
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Date: Wed, 31 Jan 2001 16:06:50 -0800 (PST)
Organization: BSD, Inc.
From: John Baldwin <jhb@osd.bsdi.com>
To: Matthew Emmerton <matt@gsicomp.on.ca>
Subject: Re: Atomic bit operations
Cc: freebsd-hackers@FreeBSD.org, John Baldwin <jhb@FreeBSD.org>
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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.

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.

> 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;

-- 

John Baldwin <jhb@osd.bsdi.com> -- http://www.FreeBSD.org/~jhb/
PGP Key: http://www.Baldwin.cx/~john/pgpkey.asc
"Power Users Use the Power to Serve!"  -  http://www.FreeBSD.org/


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