Re: ARM board lockups/hangs triggered by locks and mutexes

[Geert Uytterhoeven]

Hi Rafal,

On Mon, Aug 7, 2023 at 1:11 PM Rafał Miłecki <zajec5@xxxxxxxxx> wrote:
> On 4.08.2023 13:07, Rafał Miłecki wrote:
> > I triple checked that. Dropping a single unused function breaks kernel /
> > device stability on BCM53573!
> >
> > AFAIK the only thing below diff actually affects is location of symbols
> > (I actually verified that by comparing System.map before and after -
> > over 22'000 of relocated symbols).
> >
> > Can some unfortunate location of symbols cause those hangs/lockups?
>
> I performed another experiment. First I dropped mtd_check_of_node() to
> bring kernel back to the stable state.
>
> Then I started adding useless code to the mtdchar_unlocked_ioctl(). I
> ended up adding just enough to make sure all post-mtd symbols in
> System.map got the same offset as in case of backporting
> mtd_check_of_node().
>
> I started experiencing lockups/hangs again.
>
> I repeated the same test with adding dumb code to the brcm_nvram_probe()
> and verifying symbols offsets following brcm_nvram_probe one.
>
> I believe this confirms that this problem is about offset or alignment
> of some specific symbol(s). The remaining question is what symbols and
> how to fix or workaround that.

I had similar experiences on other ARM platforms many years ago:
bisection lead to something completely bogus, and it turned out
adding a single line of innocent code made the system lock-up or crash
unexpectedly.  It was definitely related to alignment, as adding the
right extra amount of innocent code would fix the problem. Until some
later change changing alignment again...
I never found the real cause, but the problems went away over time.
I am not sure I did enable all required errata config options, so I
may have missed some...

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds