Re: ftrace: preemptoff selftest not working

[Ingo Molnar]

>  #ifdef CONFIG_FTRACE_STARTUP_TEST
> +	/*
> +	 * When this gets called we hold the BKL which means that preemption
> +	 * is disabled. Various trace selftests however need to disable
> +	 * and enable preemption for successful tests. So we drop the BKL here
> +	 * and grab it after the tests again.
> +	 */
> +	unlock_kernel();
>  	if (type->selftest) {
>  		struct tracer *saved_tracer = current_trace;
>  		struct trace_array *tr = &global_trace;
> @@ -562,6 +569,7 @@ int register_tracer(struct tracer *type)
>  		}
>  		printk(KERN_CONT "PASSED\n");
>  	}
> +	lock_kernel();

dropping the BKL was a good idea, but the code flow was not 
investigated thoroughly enough, which caused this crash to trigger in 
-tip testing:

calling  utsname_sysctl_init+0x0/0x11 @ 1
initcall utsname_sysctl_init+0x0/0x11 returned 0 after 0 usecs
calling  init_sched_switch_trace+0x0/0xf @ 1
Testing tracer sched_switch: PASSED
initcall init_sched_switch_trace+0x0/0xf returned 0 after 101562 usecs
calling  init_stack_trace+0x0/0xf @ 1
Testing tracer sysprof: PASSED
initcall init_stack_trace+0x0/0xf returned 0 after 105468 usecs
calling  init_irqsoff_tracer+0x0/0x11 @ 1
Testing tracer irqsoff: .. no entries found ..FAILED!
initcall init_irqsoff_tracer+0x0/0x11 returned 0 after 3906 usecs
calling  init_mmio_trace+0x0/0xf @ 1
------------[ cut here ]------------
Kernel BUG at c0c0a915 [verbose debug info unavailable]
invalid opcode: 0000 [#1] SMP DEBUG_PAGEALLOC
last sysfs file: 

Pid: 1, comm: swapper Not tainted (2.6.28-rc5-tip #53704) 
EIP: 0060:[<c0c0a915>] EFLAGS: 00010286 CPU: 1
EIP is at unlock_kernel+0x10/0x2b
EAX: ffffffff EBX: 00000000 ECX: 00000000 EDX: f7030000
ESI: c12da19c EDI: 00000000 EBP: f7039f54 ESP: f7039f54
 DS: 007b ES: 007b FS: 00d8 GS: 0000 SS: 0068
Process swapper (pid: 1, ti=f7038000 task=f7030000 task.ti=f7038000)
Stack:
 f7039f6c c0164d30 c013fed8 a7d8d7b4 00000000 00000000 f7039f74 c12fb78a
 f7039fd0 c0101132 c12fb77d 00000000 6f727200 6f632072 2d206564 c1002031
 0000000f f7039fa2 f7039fb0 3531b171 00000000 00000000 0000002f c12ca480
Call Trace:
 [<c0164d30>] ? register_tracer+0x66/0x13f
 [<c013fed8>] ? ktime_get+0x19/0x1b
 [<c12fb78a>] ? init_mmio_trace+0xd/0xf
 [<c0101132>] ? do_one_initcall+0x4a/0x111
 [<c12fb77d>] ? init_mmio_trace+0x0/0xf
 [<c015c7e6>] ? init_irq_proc+0x46/0x59
 [<c12e851d>] ? kernel_init+0x104/0x152
 [<c12e8419>] ? kernel_init+0x0/0x152
 [<c01038b7>] ? kernel_thread_helper+0x7/0x10
Code: 58 14 43 75 0a b8 00 9b 2d c1 e8 51 43 7a ff 64 a1 00 a0 37 c1 89 58 14 5b 5d c3 55 64 8b 15 00 a0 37 c1 83 7a 14 00 89 e5 79 04 <0f> 0b eb fe 8b 42 14 48 85 c0 89 42 14 79 0a b8 00 9b 2d c1 e8 
EIP: [<c0c0a915>] unlock_kernel+0x10/0x2b SS:ESP 0068:f7039f54
---[ end trace a7919e7f17c0a725 ]---
Kernel panic - not syncing: Attempted to kill init!

the patch below fixes it.

	Ingo

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