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From: Pranith Kumar <bobby.prani@gmail.com>
Date: Wed, 22 Apr 2015 16:24:15 -0400
Message-ID: <CAJhHMCDRvv5PASOeX5oteOGKacUN3RiHN+whLcZk+xhGj19-4Q@mail.gmail.com>
Subject: Re: [PATCH v16] sys_membarrier(): system-wide memory barrier
 (generic, x86)
To: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: 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>,
        Stephen Hemminger <stephen@networkplumber.org>,
        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>
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On Fri, Apr 17, 2015 at 11:06 AM, Mathieu Desnoyers
<mathieu.desnoyers@efficios.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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