Fix: Disable sys_membarrier when nohz_full is enabled

compudj · paulmck · commit 907565337ebf · 2017-01-23T11:32:16.000-08:00
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 &lt;mathieu.desnoyers@efficios.com&gt;
CC: Josh Triplett &lt;josh@joshtriplett.org&gt;
CC: Steven Rostedt &lt;rostedt@goodmis.org&gt;
CC: &lt;stable@vger.kernel.org&gt;	[3.10+]
Signed-off-by: Paul E. McKenney &lt;paulmck@linux.vnet.ibm.com&gt;
Cc: Frederic Weisbecker &lt;fweisbec@gmail.com&gt;
Cc: Chris Metcalf &lt;cmetcalf@mellanox.com&gt;
Cc: Rik van Riel &lt;riel@redhat.com&gt;
Acked-by: Lai Jiangshan &lt;jiangshanlai@gmail.com&gt;
Reviewed-by: Josh Triplett &lt;josh@joshtriplett.org&gt;
diff --git 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) {
