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

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Message-ID: <87a8xzm8vg.fsf@rasmusvillemoes.dk>
Date:	Thu, 23 Apr 2015 12:33:55 +0200
From:	Rasmus Villemoes <linux@...musvillemoes.dk>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:	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>,
	"Pranith Kumar" <bobby.prani@...il.com>,
	Michael Kerrisk <mtk.manpages@...il.com>
Subject: Re: [PATCH v16] sys_membarrier(): system-wide memory barrier (generic, x86)

On Fri, Apr 17 2015, Mathieu Desnoyers <mathieu.desnoyers@...icios.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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