Re: [patch V2 24/24] cpu/hotplug: Convert hotplug locking to percpu rwsem

[Michael Ellerman <mpe@ellerman.id.au>]

Thomas Gleixner <tglx@linutronix.de> writes:

> @@ -130,6 +130,7 @@ void __static_key_slow_inc(struct static
>  	 * the all CPUs, for that to be serialized against CPU hot-plug
>  	 * we need to avoid CPUs coming online.
>  	 */
> +	lockdep_assert_hotplug_held();
>  	jump_label_lock();
>  	if (atomic_read(&key->enabled) == 0) {
>  		atomic_set(&key->enabled, -1);

I seem to be hitting this assert from the ftrace event selftests,
enabled at boot with CONFIG_FTRACE_STARTUP_TEST=y, using next-20170509
(on powerpc).

[  842.691191] Testing event rpc_call_status: 
[  842.691209] ------------[ cut here ]------------
[  842.691399] WARNING: CPU: 6 PID: 1 at ../kernel/cpu.c:234 lockdep_assert_hotplug_held+0x5c/0x70
[  842.691575] Modules linked in:
[  842.691675] CPU: 6 PID: 1 Comm: swapper/0 Tainted: G        W       4.11.0-gcc-5.4.1-next-20170509 #218
[  842.691865] task: c0000001fe780000 task.stack: c0000001fe800000
[  842.692003] NIP: c0000000000ff3dc LR: c0000000000ff3d0 CTR: c000000000218650
[  842.692166] REGS: c0000001fe8036e0 TRAP: 0700   Tainted: G        W        (4.11.0-gcc-5.4.1-next-20170509)
[  842.692343] MSR: 8000000000029033 <SF,EE,ME,IR,DR,RI,LE>
[  842.692491]   CR: 28002222  XER: 20000000
[  842.692689] CFAR: c000000000171530 SOFTE: 1 
               GPR00: c0000000000ff3d0 c0000001fe803960 c0000000012b7600 0000000000000000 
               GPR04: ffffffffffffffff 0000000000000000 c0000000fc10c0e8 0000000000000000 
               GPR08: 0000000000000000 0000000000000000 0000000000000000 c0000000f8180008 
               GPR12: 0000000000002200 c00000000fd42100 c00000000000e218 0000000000000000 
               GPR16: 0000000000000000 0000000000000000 0000000000000000 0000000000000000 
               GPR20: 0000000000000000 0000000000000000 0000000000000000 c0000000f9341610 
               GPR24: c00000000127ee48 c000000000aa49d0 000000000000000a c0000000fc3c0000 
               GPR28: c00000000117b148 c000000001264230 0000000000000000 c00000000127ee48 
[  842.694287] NIP [c0000000000ff3dc] lockdep_assert_hotplug_held+0x5c/0x70
[  842.694434] LR [c0000000000ff3d0] lockdep_assert_hotplug_held+0x50/0x70
[  842.694577] Call Trace:
[  842.694658] [c0000001fe803960] [c0000000000ff3d0] lockdep_assert_hotplug_held+0x50/0x70 (unreliable)
[  842.694876] [c0000001fe803980] [c0000000002a3754] __static_key_slow_inc+0x104/0x170
[  842.695054] [c0000001fe8039f0] [c0000000002176ac] tracepoint_probe_register_prio+0x2dc/0x390
[  842.695258] [c0000001fe803a60] [c00000000024cf50] trace_event_reg+0xe0/0x130
[  842.695434] [c0000001fe803a80] [c00000000024d5f0] __ftrace_event_enable_disable+0x270/0x3e0
[  842.695601] [c0000001fe803b10] [c000000000e20328] event_trace_self_tests+0x14c/0x350
[  842.695778] [c0000001fe803bc0] [c000000000e20774] event_trace_self_tests_init+0xc8/0xf4
[  842.695944] [c0000001fe803c30] [c00000000000d87c] do_one_initcall+0x6c/0x1d0
[  842.696113] [c0000001fe803cf0] [c000000000df462c] kernel_init_freeable+0x304/0x3e4
[  842.696282] [c0000001fe803dc0] [c00000000000e23c] kernel_init+0x2c/0x170
[  842.696460] [c0000001fe803e30] [c00000000000bdec] ret_from_kernel_thread+0x5c/0x70
[  842.696662] Instruction dump:
[  842.696763] 409e0014 38210020 e8010010 7c0803a6 4e800020 3c62ffe6 3880ffff 38634808 
[  842.697009] 480720ed 60000000 2fa30000 409effd8 <0fe00000> 38210020 e8010010 7c0803a6 
[  842.697271] ---[ end trace f68728a0d30544a1 ]---


The stupidly obvious (or perhaps obviously stupid) patch below fixes it:

diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
index daefdee9411a..5531f7ce8fa6 100644
--- a/kernel/trace/trace_events.c
+++ b/kernel/trace/trace_events.c
@@ -3241,9 +3241,19 @@ static __init void event_trace_self_tests(void)
 			continue;
 		}
 
+		get_online_cpus();
+		mutex_lock(&event_mutex);
 		ftrace_event_enable_disable(file, 1);
+		mutex_unlock(&event_mutex);
+		put_online_cpus();
+
 		event_test_stuff();
+
+		get_online_cpus();
+		mutex_lock(&event_mutex);
 		ftrace_event_enable_disable(file, 0);
+		mutex_unlock(&event_mutex);
+		put_online_cpus();
 
 		pr_cont("OK\n");
 	}

cheers