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

[Rasmus Villemoes <linux@rasmusvillemoes.dk>]

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