linux-kernel - Re: [PATCH v16] sys_membarrier(): system-wide memory barrier (generic, x86)

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Message-ID: <20150422174051.5c868437@urahara>
Date:	Wed, 22 Apr 2015 17:40:51 -0700
From:	Stephen Hemminger <stephen@...workplumber.org>
To:	Pranith Kumar <bobby.prani@...il.com>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Josh Triplett <josh@...htriplett.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Nicholas Miell <nmiell@...cast.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...hat.com>,
	Alan Cox <gnomes@...rguk.ukuu.org.uk>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	David Howells <dhowells@...hat.com>,
	Michael Kerrisk <mtk.manpages@...il.com>
Subject: Re: [PATCH v16] sys_membarrier(): system-wide memory barrier
 (generic, x86)

On Wed, 22 Apr 2015 20:37:08 -0400
Pranith Kumar <bobby.prani@...il.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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