Fix: Disable sys_membarrier when nohz_full is enabled

BugLink: http://bugs.launchpad.net/bugs/1673538

commit 9075653 upstream.

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 <[email protected]>
CC: Josh Triplett <[email protected]>
CC: Steven Rostedt <[email protected]>
Signed-off-by: Paul E. McKenney <[email protected]>
Cc: Frederic Weisbecker <[email protected]>
Cc: Chris Metcalf <[email protected]>
Cc: Rik van Riel <[email protected]>
Acked-by: Lai Jiangshan <[email protected]>
Reviewed-by: Josh Triplett <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Signed-off-by: Tim Gardner <[email protected]>
Signed-off-by: Thadeu Lima de Souza Cascardo <[email protected]>

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) {