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Date: Fri, 10 Jan 2014 14:37:10 -0500
From: Waiman Long <waiman.long@hp.com>
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To: Peter Zijlstra <peterz@infradead.org>
CC: Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@ghostprotocols.net>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Aswin Chandramouleeswaran <aswin@hp.com>,
        Scott J Norton <scott.norton@hp.com>,
        Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: SIGSEGV when using "perf record -g" with 3.13-rc* kernel
References: <52D011C9.7000209@hp.com> <20140110165822.GI7572@laptop.programming.kicks-ass.net> <20140110170223.GD8224@laptop.programming.kicks-ass.net>
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On 01/10/2014 12:02 PM, Peter Zijlstra wrote:
> On Fri, Jan 10, 2014 at 05:58:22PM +0100, Peter Zijlstra wrote:
>> On Fri, Jan 10, 2014 at 10:29:13AM -0500, Waiman Long wrote:
>>> Peter,
>>>
>>> Call Trace:
>>> <NMI>   [<ffffffff815710af>] dump_stack+0x49/0x62
>>>   [<ffffffff8104e3bc>] warn_slowpath_common+0x8c/0xc0
>>>   [<ffffffff8104e40a>] warn_slowpath_null+0x1a/0x20
>>>   [<ffffffff8105f1f1>] force_sig_info+0x131/0x140
>>>   [<ffffffff81042a4f>] force_sig_info_fault+0x5f/0x70
>>>   [<ffffffff8106d8da>] ? search_exception_tables+0x2a/0x50
>>>   [<ffffffff81043b3d>] ? fixup_exception+0x1d/0x70
>>>   [<ffffffff81042cc9>] no_context+0x159/0x1f0
>>>   [<ffffffff81042e8d>] __bad_area_nosemaphore+0x12d/0x230
>>>   [<ffffffff81042e8d>] ? __bad_area_nosemaphore+0x12d/0x230
>>>   [<ffffffff81042fa3>] bad_area_nosemaphore+0x13/0x20
>>>   [<ffffffff81578fc2>] __do_page_fault+0x362/0x480
>>>   [<ffffffff81578fc2>] ? __do_page_fault+0x362/0x480
>>>   [<ffffffff815791be>] do_page_fault+0xe/0x10
>>>   [<ffffffff81575962>] page_fault+0x22/0x30
>>>   [<ffffffff815817e4>] ? bad_to_user+0x5e/0x66b
>>>   [<ffffffff81285316>] copy_from_user_nmi+0x76/0x90
>>>   [<ffffffff81017a20>] perf_callchain_user+0xd0/0x360
>>>   [<ffffffff8111f64f>] perf_callchain+0x1af/0x1f0
>>>   [<ffffffff81117693>] perf_prepare_sample+0x2f3/0x3a0
>>>   [<ffffffff8111a2af>] __perf_event_overflow+0x10f/0x220
>>>   [<ffffffff8111ab14>] perf_event_overflow+0x14/0x20
>>>   [<ffffffff8101f69e>] intel_pmu_handle_irq+0x1de/0x3c0
>>>   [<ffffffff81008e44>] ? emulate_vsyscall+0x144/0x390
>>>   [<ffffffff81576e64>] perf_event_nmi_handler+0x34/0x60
>>>   [<ffffffff8157664a>] nmi_handle+0x8a/0x170
>>>   [<ffffffff81576848>] default_do_nmi+0x68/0x210
>>>   [<ffffffff81576a80>] do_nmi+0x90/0xe0
>>>   [<ffffffff81575c67>] end_repeat_nmi+0x1e/0x2e
>>>   [<ffffffff81008e44>] ? emulate_vsyscall+0x144/0x390
>>>   [<ffffffff81008e44>] ? emulate_vsyscall+0x144/0x390
>>>   [<ffffffff81008e44>] ? emulate_vsyscall+0x144/0x390
>>> <<EOE>>   [<ffffffff81042f7d>] __bad_area_nosemaphore+0x21d/0x230
>>>   [<ffffffff81042fa3>] bad_area_nosemaphore+0x13/0x20
>>>   [<ffffffff81578fc2>] __do_page_fault+0x362/0x480
>>>   [<ffffffff8113cfbc>] ? vm_mmap_pgoff+0xbc/0xe0
>>>   [<ffffffff815791be>] do_page_fault+0xe/0x10
>>>   [<ffffffff81575962>] page_fault+0x22/0x30
>>> ---[ end trace 037bf09d279751ec ]---
>>>
>>> So this is a double page faults. Looking at relevant changes in
>>> 3.13 kernel, I spotted the following one patch that modified the
>>> perf_callchain_user() function shown up in the stack trace above:
>>>
>> Hurm, that's an expected double fault, not something we should take the
>> process down for.
>>
>> I'll have to look at how all that works for a bit.
> How easily can you reproduce this? Could you test something like the
> below, which would allow us to take double faults from NMI context.

The error can be readily reproducible in my current setup.

> ---
>   arch/x86/mm/fault.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> index 9ff85bb8dd69..18c498d4274d 100644
> --- a/arch/x86/mm/fault.c
> +++ b/arch/x86/mm/fault.c
> @@ -641,7 +641,7 @@ no_context(struct pt_regs *regs, unsigned long error_code,
>
>   	/* Are we prepared to handle this kernel fault? */
>   	if (fixup_exception(regs)) {
> -		if (current_thread_info()->sig_on_uaccess_error&&  signal) {
> +		if (!in_nmi()&&  current_thread_info()->sig_on_uaccess_error&&  signal) {
>   			tsk->thread.trap_nr = X86_TRAP_PF;
>   			tsk->thread.error_code = error_code | PF_USER;
>   			tsk->thread.cr2 = address;

Yes, this change fixed the error that I got. I no longer see SIGSEGV 
when I run the test.

I did tried to back out your "perf: Fix arch_perf_out_copy_user default" 
patch, but it didn't fix the problem.

Thank for the quick turnaround!

-Longman