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SubjectRe: [RFC PATCH] x86/syscalls: allow tracing of __do_sys_[syscall] functions
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On Sep 20, 2022, at 4:02 AM, Peter Zijlstra <peterz@infradead.org> wrote:

> On Mon, Sep 19, 2022 at 07:35:42PM -0700, Nadav Amit wrote:
>
>> 1. What is the reason that inline functions are marked with notrace?
>
> IIRC the concern is that a notrace function using an inline function;
> GCC deciding to not inline and then still hitting tracing.
>
> For noinstr we've mandated __always_inline to avoid this problem. The
> direct advantage is that those inlined into instrumented code get, well,
> instrumented.

I fully understand the __always_inline. I do not understand the inline,
which is a hint. Anyhow, I just thought that you would probably know, but
I’ll do the digging and look at the tables to see how they look with and
without inline implying notrace.

>
>> 2. Is probing function that is called from do_idle() supposed to work, or
>> should the kernel prevent it?
>
> Should work for some :-) Specifically it doesn't work for those that
> disable RCU, and that's (largely) being fixed here:
>
> https://lore.kernel.org/all/20220919095939.761690562@infradead.org/T/#u
>
> Although looking at it just now, I think I missed a spot.. lemme go fix
> ;-)
>

Thank you. I’ll give it a spin as soon as I finish some stuff (which can
be days).


> I'm failing to find this callchain; where is
> tick_nohz_get_sleep_length() calling to elfcorehdr_read() ?!?

Very strange. According to DWARF and disassembly, the call in the code is
actually to hrtimer_next_event_without() and nothing more, and
elfcorehdr_read+0x40/0x40 is actually after the ret.

The strangest part is that I actually collected additional similar crashes,
and I only now notice that all of them have elfcorehdr_read(). Good catch!
(which makes no sense)…

I’ll move to a newer kernel, apply your patches and dig into it too.

Thanks again,
Nadav



>> [ 2381.892478] elfcorehdr_read+0x40/0x40
>> [ 2381.896681] tick_nohz_get_sleep_length+0x9d/0xc0
>> [ 2381.901955] menu_select+0x4bb/0x630
>> [ 2381.905965] cpuidle_select+0x16/0x20
>> [ 2381.910069] do_idle+0x1d2/0x270
>> [ 2381.913689] cpu_startup_entry+0x20/0x30
>> [ 2381.918086] start_secondary+0x118/0x150
>> [ 2381.922484] secondary_startup_64_no_verify+0xc3/0xcb
>> [ 2381.928147] </TASK>
>> [ 2381.931535] Modules linked in: zram
>> [ 2381.936365] CR2: ffffc90077cb6e4b
>> [ 2381.940998] ---[ end trace 0000000000000000 ]—


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