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

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Date:	Wed, 22 Apr 2015 16:24:15 -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 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?)
...

> +SYSCALL_DEFINE2(membarrier, int, cmd, int, flags)
> +{
> +       switch (cmd) {
> +       case MEMBARRIER_CMD_QUERY:
> +               return MEMBARRIER_CMD_BITMASK;
> +       case MEMBARRIER_CMD_SHARED:
> +               if (num_online_cpus() > 1)
> +                       synchronize_sched();
> +               return 0;
> +       default:
> +               return -EINVAL;
> +       }
> +}



-- 
Pranith
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