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

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Date:	Wed, 22 Apr 2015 20:37:08 -0400
From:	Pranith Kumar <bobby.prani@...il.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:	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>,
	Stephen Hemminger <stephen@...workplumber.org>,
	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, Apr 22, 2015 at 4:40 PM, Mathieu Desnoyers
<mathieu.desnoyers@...icios.com> wrote:
> ----- Original Message -----
>> On Fri, Apr 17, 2015 at 11:06 AM, Mathieu Desnoyers
>> <mathieu.desnoyers@...icios.com> wrote:
>> > diff --git a/init/Kconfig b/init/Kconfig
>> > index f5dbc6d..89bad6a 100644
>> > --- a/init/Kconfig
>> > +++ b/init/Kconfig
>> > @@ -1559,6 +1559,19 @@ config PCI_QUIRKS
>> >           bugs/quirks. Disable this only if your target machine is
>> >           unaffected by PCI quirks.
>> >
>> > +config MEMBARRIER
>> > +       bool "Enable membarrier() system call" if EXPERT
>> > +       default y
>> > +       depends on SMP
>> > +       help
>> > +         Enable the membarrier() system call that allows issuing memory
>> > +         barriers across all running threads, which can be used to
>> > distribute
>> > +         the cost of user-space memory barriers asymmetrically by
>> > transforming
>> > +         pairs of memory barriers into pairs consisting of membarrier()
>> > and a
>> > +         compiler barrier.
>> > +
>> > +         If unsure, say Y.
>> > +
>>
>> 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.

-- 
Pranith
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