linux-kernel - Re: [PATCH v2 07/15] x86/lib/copy_user_64.S: Convert to ALTERNATIVE

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Date:	Thu, 5 Mar 2015 10:46:50 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Borislav Petkov <bp@...en8.de>, Jiri Olsa <jolsa@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Adrian Hunter <adrian.hunter@...el.com>
Cc:	X86 ML <x86@...nel.org>, Andy Lutomirski <luto@...capital.net>,
	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v2 07/15] x86/lib/copy_user_64.S: Convert to ALTERNATIVE_2


* Ingo Molnar <mingo@...nel.org> wrote:

> Ha! There's a neat alternatives debugging trick with perf that you 
> might not know about: if you run 'perf top' as root then perf will 
> use /proc/kcore to disassemble the live kernel image and if you look 
> at the assembly output of hot functions then you'll see the real, 
> patched instructions on the live kernel, not the vmlinux 
> instructions.
> 
> I have two enhancement suggestions to the perf tooling developers for 
> this usecase:

and I've got a bugreport as well, when I use the kcore annotations on 
'fput' symbol (i.e. running 'perf top' as root), go into the 
disassembly window and then try to exit that window, then I get this 
segfault:

Program received signal SIGSEGV, Segmentation fault.                                                                                                                     
[Switching to Thread 0x7ffff3c41700 (LWP 29134)]                                                                                                                         
lock__delete (ops=0x7fffec001818) at util/annotate.c:219
219                     ins__delete(ops->locked.ops);
(gdb) bt
#0  lock__delete (ops=0x7fffec001818) at util/annotate.c:219
#1  0x000000000046e568 in disasm_line__free (dl=0x7fffec0017e0) at util/annotate.c:619
#2  0x00000000004f0743 in symbol__tui_annotate (sym=<optimized out>, map=<optimized out>, evsel=evsel@...ry=0x83d770, hbt=hbt@...ry=0x7ffff3c40e60)
    at ui/browsers/annotate.c:976
#3  0x00000000004f0973 in hist_entry__tui_annotate (he=he@...ry=0x151ba40, evsel=evsel@...ry=0x83d770, hbt=hbt@...ry=0x7ffff3c40e60) at ui/browsers/annotate.c:835
#4  0x00000000004f5685 in perf_evsel__hists_browse (evsel=0x83d770, nr_events=nr_events@...ry=1, 
    helpline=helpline@...ry=0x51d350 "For a higher level overview, try: perf top --sort comm,dso", left_exits=left_exits@...ry=false, hbt=hbt@...ry=0x7ffff3c40e60, 
    min_pcnt=<optimized out>, env=0x83de40) at ui/browsers/hists.c:1716
#5  0x00000000004f7ae4 in perf_evlist__tui_browse_hists (evlist=0x7cbb90, help=help@...ry=0x51d350 "For a higher level overview, try: perf top --sort comm,dso", 
    hbt=hbt@...ry=0x7ffff3c40e60, min_pcnt=<optimized out>, env=<optimized out>) at ui/browsers/hists.c:2006
#6  0x0000000000435850 in display_thread_tui (arg=0x7fffffffa580) at builtin-top.c:582
#7  0x00007ffff7bc40a5 in start_thread (arg=0x7ffff3c41700) at pthread_create.c:309
#8  0x00007ffff5fe4cfd in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:111

using latestest tools/perf.

Thanks,

	Ingo
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