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

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Date:	Tue, 21 Apr 2015 13:47:38 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
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>,
	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, 17 Apr 2015, Mathieu Desnoyers wrote:
> +/**
> + * enum membarrier_cmd - membarrier system call command
> + *

    * @MEMBARRIER_CMD_QUERY:	Explanatory blurb......

Hint: Create a kerneldoc document and build it.

> + * Command to be passed to the membarrier system call.

Please epxlain, that the commands need to be a single bit each.

> +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.

Is it really worth to make this configurable?

> +/**
> + * sys_membarrier - issue memory barriers on a set of threads
> + * @cmd:   MEMBARRIER_CMD_QUERY:
> + *             Query the set of supported commands. It returns a bitmask of
> + *             supported commands.
> + *         MEMBARRIER_CMD_SHARED:
> + *             Execute a memory barrier on all running threads. Upon
> + *             return from system call, the caller thread is ensured that
> + *             all running threads have passed through a state where all
> + *             memory accesses to user-space addresses match program order
> + *             between entry to and return from the system call (non-running
> + *             threads are de facto in such a state). This covers threads
> + *             from all processes running on the system. This command
> + *             returns 0.

I think the explanation for the commands should be in the enum
documentation. This here should explain that @cmd takes command values
defined in the enum.

> +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;
> +	}
> +}

This looks way cleaner now :)

Thanks,

	tglx
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