[PATCH v3 tip/core/rcu 03/18] Fix: Disable sys_membarrier when nohz_full is enabled

From: Paul E. McKenney
Date: Tue Jan 24 2017 - 16:52:58 EST

From: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>

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

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: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
CC: Steven Rostedt <rostedt@xxxxxxxxxxx>
CC: <stable@xxxxxxxxxxxxxxx> [3.10+]
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: Frederic Weisbecker <fweisbec@xxxxxxxxx>
Cc: Chris Metcalf <cmetcalf@xxxxxxxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxx>
Acked-by: Lai Jiangshan <jiangshanlai@xxxxxxxxx>
Reviewed-by: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
kernel/membarrier.c | 4 ++++
1 file changed, 4 insertions(+)

diff --git a/kernel/membarrier.c b/kernel/membarrier.c
index 536c727a56e9..9f9284f37f8d 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) {