From: Rasmus Villemoes <linux@rasmusvillemoes.dk>
To: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: 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>,
        "Pranith Kumar" <bobby.prani@gmail.com>,
        Michael Kerrisk <mtk.manpages@gmail.com>
Subject: Re: [PATCH v16] sys_membarrier(): system-wide memory barrier (generic, x86)
Organization: D03
References: <1429283202-6245-1-git-send-email-mathieu.desnoyers@efficios.com>
Date: Thu, 23 Apr 2015 12:33:55 +0200
In-Reply-To: <1429283202-6245-1-git-send-email-mathieu.desnoyers@efficios.com>
	(Mathieu Desnoyers's message of "Fri, 17 Apr 2015 11:06:42 -0400")
Message-ID: <87a8xzm8vg.fsf@rasmusvillemoes.dk>
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On Fri, Apr 17 2015, Mathieu Desnoyers <mathieu.desnoyers@efficios.com> wrote:

> + */
> +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;
> +	}
> +}

Shouldn't flags be enforced 0, to actually make future extensions
possible without risk of breaking some sloppy userspace? I think that is
or should be part of "make sure new syscalls take a flags parameter".

> + * If this system call is not implemented, -ENOSYS is returned. If the
> + * command specified does not exist, or if the command argument is invalid,
> + * this system call returns -EINVAL. For a given command, this system call
> + * is guaranteed to always return the same value until reboot.

I like that guarantee, but it may be a bit much to promise for any and
all possible future flags. So maybe weaken it to 'For a given command
and flags==0, this ...'.


Rasmus
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