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Date:   Tue, 31 Aug 2021 17:18:30 +0200
From:   Heiko Carstens <hca@linux.ibm.com>
To:     Marco Elver <elver@google.com>
Cc:     Christian Borntraeger <borntraeger@de.ibm.com>,
        Nathan Chancellor <nathan@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Vasily Gorbik <gor@linux.ibm.com>, linux-s390@vger.kernel.org,
        linux-kernel@vger.kernel.org, llvm@lists.linux.dev,
        qemu-s390x <qemu-s390x@nongnu.org>
Subject: Re: [GIT PULL] s390 updates for 5.15 merge window
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On Tue, Aug 31, 2021 at 05:02:15PM +0200, Marco Elver wrote:
> On Tue, Aug 31, 2021 at 12:46PM +0200, Marco Elver wrote:
> > On Tue, 31 Aug 2021 at 12:13, Heiko Carstens <hca@linux.ibm.com> wrote:
> I repro'd, and the problem is part QEMU TCG and a minor problem with
> stack_trace_save() on s390:
> 
> 1. QEMU TCG doesn't seem to want to execute threads concurrently,
>    resulting in no "value changes" being observed. This is probably just
>    a limitation of TCG, and if run on a real CPU, shouldn't be a problem.
>    On QEMU, most test cases will pass with CONFIG_KCSAN_REPORT_VALUE_CHANGE_ONLY=n
>    (There's one left that requires value changes to be observable)
> 
> 2. stack_trace_save() is subtly broken on s390: it starts the trace in
>    stack_trace_save() itself. This is incorrect, as the trace should
>    start with the caller. We reported something similar to arm64, also
>    because one of our sanitizer tests failed:
>    https://lkml.kernel.org/r/20210319184106.5688-1-mark.rutland@arm.com
...
> I fixed it with the below, and now most tests pass. Note that, other
> debugging tools may also report misleading stack traces without the
> stack_trace_save() fix (e.g. certain KFENCE reports).
> 
> If you have a better solution for how to fix stack_trace_save() on s390,
> please discard my patch.

Ah, with your patch I get the below on a real machine; so it looks
like this really was the only problem for the failing self tests.

Thanks a lot for the patch! Will look into it later and apply if
everything else still works!

[    5.138547]     ok 1 - test_basic
[    7.538799]     ok 2 - test_concurrent_races
[    9.938524]     ok 3 - test_novalue_change
[   11.620063]     ok 4 - test_novalue_change_exception
[   11.952404]     ok 5 - test_unknown_origin
[   14.349852]     ok 6 - test_write_write_assume_atomic
[   14.681937]     ok 7 - test_write_write_struct
[   15.031046]     ok 8 - test_write_write_struct_part
[   17.429869]     ok 9 - test_read_atomic_write_atomic
[   17.760862]     ok 10 - test_read_plain_atomic_write
[   18.093500]     ok 11 - test_read_plain_atomic_rmw
[   20.490570]     ok 12 - test_zero_size_access
[   22.939270]     ok 13 - test_data_race
[   23.271104]     ok 14 - test_assert_exclusive_writer
[   23.601940]     ok 15 - test_assert_exclusive_access
[   26.000281]     ok 16 - test_assert_exclusive_access_writer
[   26.330718]     ok 17 - test_assert_exclusive_bits_change
[   28.739480]     ok 18 - test_assert_exclusive_bits_nochange
[   28.962218]     ok 19 - test_assert_exclusive_writer_scoped
[   29.071385]     ok 20 - test_assert_exclusive_access_scoped
[   31.479706]     ok 21 - test_jiffies_noreport
[   33.879581]     ok 22 - test_seqlock_noreport
[   36.279853]     ok 23 - test_atomic_builtins
[   36.279893] ok 1 - kcsan