On Fri, Nov 4, 2016 at 12:29 AM, Mathieu Desnoyers
<mathieu.desnoyers@xxxxxxxxxxxx> 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>
> 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;
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@xxxxxxxxx>
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
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