Date: Thu, 26 Jul 2001 14:43:24 -0700 (PDT) From: John Baldwin <jhb@FreeBSD.org> To: Arun Sharma <arun@sharmas.dhs.org> Cc: hackers@FreeBSD.org Subject: RE: Need a clean room implementation of this function Message-ID: <XFMail.010726144324.jhb@FreeBSD.org> In-Reply-To: <20010726135913.A23052@sharmas.dhs.org>
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On 26-Jul-01 Arun Sharma wrote:
> I'm porting a BSD licensed Java VM from Linux to FreeBSD and ran into
> the following Linux function which is not implemented in BSDs.
>
> To avoid GPL contamination issues, can someone complete[1] the following
> method in inlined IA-32 assembly ? Intel instruction reference documents
> an instruction called BTS, which does just this.
You might be able to rewrite the code to use atomic_cmpset() instead of
test_and_set_bit() (which you can steal from machine/atomic.h in -current).
You can emulate this exact operation with a loop using atomic_cmpset* if
needed:
{
int val;
do {
val = *(int *)addr;
} while (atomic_cmpset_int(addr, val, val | (1 << nr) == 0);
return (val & (1 << nr));
}
If the code in question is just using a single bit in a word as a spinlock like
so:
while (test_and_set_bit(0, &lock_var))
;
then you can replace that with an atomic_cmpset loop:
while (atomic_cmpset_int(&lock_var, 0, 1) == 0)
;
However, you will want to use appropriate memory barriers for this to work well
on SMP systems (and if you want this portable to other archs at all). In that
case you should use an 'acquire' barrier to acquire the lock, and a 'release'
barrier to release the lock. Thus:
/* lock critical region */
while (atomic_cmpset_acq_int(&lock_var, 0, 1) == 0)
; /* spin */
...
/* release lock */
atomic_store_rel_int(&lock_var, 0);
--
John Baldwin <jhb@FreeBSD.org> -- 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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