Date: Fri, 7 Apr 2000 08:34:31 +0100 (BST) From: Doug Rabson <dfr@nlsystems.com> To: Matthew Dillon <dillon@apollo.backplane.com> Cc: Alfred Perlstein <bright@wintelcom.net>, Greg Lehey <grog@lemis.com>, current@freebsd.org Subject: Re: cvs commit: src/sys/i386/i386 support.s src/sys/kern init_sysent.c kern_prot.c kern_sig.c Message-ID: <Pine.BSF.4.21.0004070833040.250-100000@salmon.nlsystems.com> In-Reply-To: <200004061757.KAA89372@apollo.backplane.com>
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On Thu, 6 Apr 2000, Matthew Dillon wrote: > > :The version of Linux kernel source that I have uses the first: > : > :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 __SMP__ > :{ > : struct task_struct *old = parent; > : mb(); > : parent = me->p_opptr; > : if (old != parent) > : continue; > :} > :#endif > : break; > : } > : return pid; > :} > : > :I like it. mb() is most certainly a "memory barrier" inline to > :force ordering constraints. interesting how they don't use > :volatile for the pointer though: > > mb() just prevents the compiler from optimizing access to the > structural fields (otherwise it might move the accesses outside > the for() loop and you would get an infinite loop. From the > compiler's point of view, mb() is a subroutine call (I assume > in the headers it's a volatile __asm). > > We can either use an mb() type of thing, or we can declare the structural > field volatile, or we can cast the access to be volatile. It also forces the cpu to drain writes and prevents reads from being re-ordered before the mb(). This is mainly a hint for alpha processors but I think it is relavent to other architectures. -- Doug Rabson Mail: dfr@nlsystems.com Nonlinear Systems Ltd. Phone: +44 181 442 9037 To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-current" in the body of the message
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