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

[Paul E. McKenney]

On Mon, Nov 07, 2016 at 05:08:59PM +0000, Mathieu Desnoyers wrote:
> ----- On Nov 3, 2016, at 1:49 PM, Paul E. McKenney paulmck@xxxxxxxxxxxxxxxxxx wrote:
> 
> > 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>
> 
> Hi Paul,
> 
> Do you plan to pick it up through your tree, or I should sent
> it directly to Linus ?

Your choice.  I believe that the original went some other way, but I
would be fine carrying this one.

							Thanx, Paul

> Thanks,
> 
> Mathieu
> 
> > 
> >> 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
> 
> -- 
> Mathieu Desnoyers
> EfficiOS Inc.
> http://www.efficios.com
>