Date: Wed, 26 Mar 2003 12:13:21 -0500 (EST) From: John Baldwin <jhb@FreeBSD.org> To: Craig Rodrigues <rodrigc@attbi.com> Cc: freebsd-smp@FreeBSD.org Subject: Re: atomic_dec_and_test() in FreeBSD? Message-ID: <XFMail.20030326121321.jhb@FreeBSD.org> In-Reply-To: <20030326023737.GA85101@attbi.com>
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On 26-Mar-2003 Craig Rodrigues wrote: > On Mon, Mar 24, 2003 at 11:48:10AM -0500, John Baldwin wrote: >> > http://kernelnewbies.org/documents/kdoc/kernel-api/r287.html >> >> If you strictly need decrement and test, you can use a while loop >> with atomic_cmpset(), e.g. >> >> do { >> x = foo; >> } while (atomic_cmpset_int(&foo, x, x - 1) == 0); >> if (x == 1) >> /* foo just dropped to zero */ >> >> What task are you trying to accomplish though? > > > I am the port maintainer of the Apache Portable Runtime (apr) library. > apr has some atomic functions. For FreeBSD, this is what is defined: > > /** > * decrement the atomic variable by 1 > * @param mem pointer to the atomic value > * @return zero if the value is zero, otherwise non-zero > */ > int apr_atomic_dec(volatile apr_atomic_t *mem); > > [snip] > >#define apr_atomic_dec(mem) atomic_subtract_int(mem,1) > > > This is obviously quite wrong. > > So are you saying that I should replace this with: > > int apr_atomic_dec(volatile apr_atomic_t *mem); > > int apr_atomic_dec(volatile apr_atomic_t *mem){ > apr_atomic_t x > do { > x = *mem; > } while (atomic_cmpset_int(mem, x, x - 1) == 0); > if (x == 1) > /* foo just dropped to zero */ > > > ??????? > } > > Can you give more guidance? You could do this: apr_atomic_t x; do { x = *mem; } while (atomic_cmpset_int(mem, x, x - 1) == 0); return (x - 1) -- John Baldwin <jhb@FreeBSD.org> <>< http://www.FreeBSD.org/~jhb/ "Power Users Use the Power to Serve!" - http://www.FreeBSD.org/
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