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

[Paul E. McKenney]

On Thu, Nov 03, 2016 at 10:29:28AM -0600, Mathieu Desnoyers 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@xxxxxxxxxxxx>
> CC: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>

Acked-by: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>

> CC: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
> CC: Steven Rostedt <rostedt@xxxxxxxxxxx>
> CC: Lai Jiangshan <jiangshanlai@xxxxxxxxx>
> CC: <stable@xxxxxxxxxxxxxxx>	[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;
>  	if (unlikely(flags))
>  		return -EINVAL;
>  	switch (cmd) {
> -- 
> 2.1.4
>