Re: [PATCH] Fix: disable sys_membarrier when nohz_full is enabled

[Lai Jiangshan <jiangshanlai@gmail.com>]

On Fri, Nov 4, 2016 at 12:29 AM, Mathieu Desnoyers
<mathieu.desnoyers@efficios.com> wrote:
> Userspace applications should be allowed to expect the membarrier system
> call with MEMBARRIER_CMD_SHARED command to issue memory barriers on
> nohz_full CPUs, but synchronize_sched() does not take those into
> account.
>
> Given that we do not want unrelated processes to be able to affect
> real-time sensitive nohz_full CPUs, simply return ENOSYS when membarrier
> is invoked on a kernel with enabled nohz_full CPUs.
>
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> CC: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> CC: Josh Triplett <josh@joshtriplett.org>
> CC: Steven Rostedt <rostedt@goodmis.org>
> CC: Lai Jiangshan <jiangshanlai@gmail.com>
> CC: <stable@vger.kernel.org>    [3.10+]
> ---
>  kernel/membarrier.c | 4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/kernel/membarrier.c b/kernel/membarrier.c
> index 536c727..9f9284f 100644
> --- a/kernel/membarrier.c
> +++ b/kernel/membarrier.c
> @@ -16,6 +16,7 @@
>
>  #include <linux/syscalls.h>
>  #include <linux/membarrier.h>
> +#include <linux/tick.h>
>
>  /*
>   * Bitmask made from a "or" of all commands within enum membarrier_cmd,
> @@ -51,6 +52,9 @@
>   */
>  SYSCALL_DEFINE2(membarrier, int, cmd, int, flags)
>  {
> +       /* MEMBARRIER_CMD_SHARED is not compatible with nohz_full. */
> +       if (tick_nohz_full_enabled())
> +               return -ENOSYS;

I guess this code needs to be moved down into the branch of
"case MEMBARRIER_CMD_SHARED" to match its comment.

Acked-by: Lai Jiangshan <jiangshanlai@gmail.com>

But I'm afraid, in the future, tick_nohz_full will become a default y
feature. thus it makes sys_membarrier() always disabled. we might
need a new MEMBARRIER_CMD_XXX to handle it?

thanks,
Lai