[PATCH 3.16.y-ckt 017/142] tracepoints: Do not trace when cpu is offline

From: Luis Henriques
Date: Tue Mar 22 2016 - 07:21:34 EST

3.16.7-ckt26 -stable review patch. If anyone has any objections, please let me know.


From: "Steven Rostedt (Red Hat)" <rostedt@xxxxxxxxxxx>

commit f37755490fe9bf76f6ba1d8c6591745d3574a6a6 upstream.

The tracepoint infrastructure uses RCU sched protection to enable and
disable tracepoints safely. There are some instances where tracepoints are
used in infrastructure code (like kfree()) that get called after a CPU is
going offline, and perhaps when it is coming back online but hasn't been
registered yet.

This can probuce the following warning:

[ INFO: suspicious RCU usage. ]
4.4.0-00006-g0fe53e8-dirty #34 Tainted: G S
include/trace/events/kmem.h:141 suspicious rcu_dereference_check() usage!

other info that might help us debug this:

RCU used illegally from offline CPU! rcu_scheduler_active = 1, debug_locks = 1
no locks held by swapper/8/0.

stack backtrace:
CPU: 8 PID: 0 Comm: swapper/8 Tainted: G S 4.4.0-00006-g0fe53e8-dirty #34
Call Trace:
[c0000005b76c78d0] [c0000000008b9540] .dump_stack+0x98/0xd4 (unreliable)
[c0000005b76c7950] [c00000000010c898] .lockdep_rcu_suspicious+0x108/0x170
[c0000005b76c79e0] [c00000000029adc0] .kfree+0x390/0x440
[c0000005b76c7a80] [c000000000055f74] .destroy_context+0x44/0x100
[c0000005b76c7b00] [c0000000000934a0] .__mmdrop+0x60/0x150
[c0000005b76c7b90] [c0000000000e3ff0] .idle_task_exit+0x130/0x140
[c0000005b76c7c20] [c000000000075804] .pseries_mach_cpu_die+0x64/0x310
[c0000005b76c7cd0] [c000000000043e7c] .cpu_die+0x3c/0x60
[c0000005b76c7d40] [c0000000000188d8] .arch_cpu_idle_dead+0x28/0x40
[c0000005b76c7db0] [c000000000101e6c] .cpu_startup_entry+0x50c/0x560
[c0000005b76c7ed0] [c000000000043bd8] .start_secondary+0x328/0x360
[c0000005b76c7f90] [c000000000008a6c] start_secondary_prolog+0x10/0x14

This warning is not a false positive either. RCU is not protecting code that
is being executed while the CPU is offline.

Instead of playing "whack-a-mole(TM)" and adding conditional statements to
the tracepoints we find that are used in this instance, simply add a
cpu_online() test to the tracepoint code where the tracepoint will be
ignored if the CPU is offline.

Use of raw_smp_processor_id() is fine, as there should never be a case where
the tracepoint code goes from running on a CPU that is online and suddenly
gets migrated to a CPU that is offline.

Link: http://lkml.kernel.org/r/1455387773-4245-1-git-send-email-kda@xxxxxxxxxxxxxxxxx

Reported-by: Denis Kirjanov <kda@xxxxxxxxxxxxxxxxx>
Fixes: 97e1c18e8d17b ("tracing: Kernel Tracepoints")
Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
[ luis: backported to 3.16:
- included linux/percpu.h as suggested by Steven for other stable kernels ]
Signed-off-by: Luis Henriques <luis.henriques@xxxxxxxxxxxxx>
include/linux/tracepoint.h | 6 ++++++
1 file changed, 6 insertions(+)

diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
index 2e2a5f7717e5..30e788953524 100644
--- a/include/linux/tracepoint.h
+++ b/include/linux/tracepoint.h
@@ -14,8 +14,11 @@
* See the file COPYING for more details.

+#include <linux/smp.h>
#include <linux/errno.h>
#include <linux/types.h>
+#include <linux/percpu.h>
+#include <linux/cpumask.h>
#include <linux/rcupdate.h>
#include <linux/static_key.h>

@@ -121,6 +124,9 @@ extern void syscall_unregfunc(void);
void *it_func; \
void *__data; \
+ if (!cpu_online(raw_smp_processor_id())) \
+ return; \
+ \
if (!(cond)) \
return; \
prercu; \