Re: SIGSEGV when using "perf record -g" with 3.13-rc* kernel

[Peter Zijlstra]

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.

---
 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;
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