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Date:   Tue, 24 Jan 2017 13:51:14 -0800
From:   "Paul E. McKenney" <>
Cc:,,,,,,,,,,,,,,, <>,
        "Paul E. McKenney" <>,
        Chris Metcalf <>,
        Rik van Riel <>
Subject: [PATCH v3 tip/core/rcu 03/18] Fix: Disable sys_membarrier when nohz_full is enabled

From: Mathieu Desnoyers <>

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 <>
CC: Josh Triplett <>
CC: Steven Rostedt <>
CC: <>	[3.10+]
Signed-off-by: Paul E. McKenney <>
Cc: Frederic Weisbecker <>
Cc: Chris Metcalf <>
Cc: Rik van Riel <>
Acked-by: Lai Jiangshan <>
Reviewed-by: Josh Triplett <>
 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) {

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