4.4-stable review patch. If anyone has any objections, please let me know.
------------------ From: Mathieu Desnoyers <[email protected]> commit 907565337ebf998a68cb5c5b2174ce5e5da065eb 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]> --- kernel/membarrier.c | 4 ++++ 1 file changed, 4 insertions(+) --- 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) {
