Date: Wed, 22 Apr 2015 17:40:51 -0700
From: Stephen Hemminger <stephen@networkplumber.org>
To: Pranith Kumar <bobby.prani@gmail.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Josh Triplett <josh@joshtriplett.org>,
        KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Nicholas Miell <nmiell@comcast.net>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Ingo Molnar <mingo@redhat.com>, Alan Cox <gnomes@lxorguk.ukuu.org.uk>,
        Lai Jiangshan <laijs@cn.fujitsu.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        David Howells <dhowells@redhat.com>,
        Michael Kerrisk <mtk.manpages@gmail.com>
Subject: Re: [PATCH v16] sys_membarrier(): system-wide memory barrier
 (generic, x86)
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On Wed, 22 Apr 2015 20:37:08 -0400
Pranith Kumar <bobby.prani@gmail.com> wrote:

> >> I understand why this syscall makes sense on SMP only, but you are
> >> anyways checking num_online_cpus() and returning if it is only one. Is
> >> this limitation necessary then? How do !SMP systems handle this
> >> syscall? (I am guessing glibc wrapper?)  
> >
> > For !SMP, this system call is not implemented (returns -ENOSYS).
> > Userspace libs are expected to query sysconf(_SC_NPROCESSORS_CONF)
> > and check whether the system supports multiprocessor at all. If
> > only a single processor is supported by the kernel, then userspace
> > can skip the calls to sys_membarrier altogether, because they are
> > not even needed.
> >
> > Do you think this kind of information belongs in a man page ?
> >
> > Should we instead just implement the system call in !SMP, and
> > return 0 without any side-effect ? This would be a bit inefficient
> > to let userspace call a system call that has no effect whatsoever.
> >  
> 
> Are there any other SMP-only system calls like this? I am not really
> sure what is the right way but documenting it would be good.


The syscall should just return 0.
Let the application not worry about how many CPU's are present
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