Date: Tue, 3 Oct 2000 12:16:04 -0700 From: Alfred Perlstein <bright@wintelcom.net> To: Chuck Paterson <cp@bsdi.com> Cc: John Baldwin <jhb@FreeBSD.ORG>, arch@FreeBSD.ORG, John Polstra <jdp@polstra.com>, Daniel Eischen <eischen@vigrid.com>, Matt Dillon <dillon@earth.backplane.com>, Greg Lehey <grog@lemis.com> Subject: Re: Mutexes and semaphores Message-ID: <20001003121604.H27736@fw.wintelcom.net> In-Reply-To: <200010031528.JAA10873@berserker.bsdi.com>; from cp@bsdi.com on Tue, Oct 03, 2000 at 09:28:39AM -0600 References: <XFMail.001003010402.jhb@FreeBSD.org> <200010031528.JAA10873@berserker.bsdi.com>
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> On 27-Sep-00 Alfred Perlstein wrote:
> > Right now I can't even do getpid() properly because we don't have
> > read/write-barriers.
>
> We do have these. I have some helper functions I'll try to run by the
> new-bus list tonight.
>
* Chuck Paterson <cp@bsdi.com> [001003 08:28] wrote:
> Why do yo need a barier for getpid()?
> A barrier really is not guaranteed to be sufficient
> for the
>
> #if defined(COMPAT_43) || defined(COMPAT_SUNOS)
> p->p_retval[1] = p->p_pptr->p_pid;
> #endif
From linux/kernel/timer.c:
(It assumes struct proc is in stable storage)
/*
* This is not strictly SMP safe: p_opptr could change
* from under us. However, rather than getting any lock
* we can use an optimistic algorithm: get the parent
* pid, and go back and check that the parent is still
* the same. If it has changed (which is extremely unlikely
* indeed), we just try again..
*
* NOTE! This depends on the fact that even if we _do_
* get an old value of "parent", we can happily dereference
* the pointer: we just can't necessarily trust the result
* until we know that the parent pointer is valid.
*
* The "mb()" macro is a memory barrier - a synchronizing
* event. It also makes sure that gcc doesn't optimize
* away the necessary memory references.. The barrier doesn't
* have to have all that strong semantics: on x86 we don't
* really require a synchronizing instruction, for example.
* The barrier is more important for code generation than
* for any real memory ordering semantics (even if there is
* a small window for a race, using the old pointer is
* harmless for a while).
*/
asmlinkage long sys_getppid(void)
{
int pid;
struct task_struct * me = current;
struct task_struct * parent;
parent = me->p_opptr;
for (;;) {
pid = parent->pid;
#if CONFIG_SMP
{
struct task_struct *old = parent;
mb();
parent = me->p_opptr;
if (old != parent)
continue;
}
#endif
break;
}
return pid;
}
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