Re: [PATCH] ARM: return_address: disable again for CONFIG_ARM_UNWIND=y

Le Sat, Mar 05, 2022 at 10:04:23PM +0000, Ard Biesheuvel a écrit :

On Sat, 5 Mar 2022 at 20:20, Corentin Labbe [off-list ref] wrote:

quoted

Le Wed, Mar 02, 2022 at 12:32:01PM +0100, Ard Biesheuvel a écrit :

quoted

Commit 41918ec82eb6 ("ARM: ftrace: enable the graph tracer with the EABI
unwinder") removed the dummy version of return_address() that was
provided for the CONFIG_ARM_UNWIND=y case, on the assumption that the
removal of the kernel_text_address() call from unwind_frame() in the
preceding patch made it safe to do so.

However, this turns out not to be the case: Corentin reports warnings
about suspicious RCU usage and other strange behavior that seems to
originate in the stack unwinding that occurs in return_address().

Given that the function graph tracer (which is what these changes were
enabling for CONFIG_ARM_UNWIND=y builds) does not appear to care about
this distinction, let's revert return_address() to the old state.

Cc: Corentin Labbe <clabbe.montjoie@gmail.com>
Fixes: 41918ec82eb6 ("ARM: ftrace: enable the graph tracer with the EABI unwinder")
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

Hello

On next-20220304 even with both
ARM: unwind: avoid spurious warnings on bogus code addresses
and
ARM: return_address: disable again for CONFIG_ARM_UNWIND=y

I got:
[    0.134043] =============================
[    0.138372] WARNING: suspicious RCU usage
[    0.142702] 5.17.0-rc6-next-20220304-dirty #3 Not tainted

...

quoted

[    1.015443]  show_stack from 0xf0991e6c

On next-20220303 I got nothing.

This makes me suspect that this is unrelated, given that no changes
were applied to Russell's tree in the mean time, as far as I can tell.

Hello

I bisected to dc6e0818bc9a0336d9accf3ea35d146d72aa7a18 ("sched/cpuacct: Optimize away RCU read lock")
but reverting lead to some remaining RCU (but far less than before).

I continue to debug it.

Regards

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