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

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Message-ID: <CAMuHMdXPB+nSHUTPufiaRP0cxvk2_raEHavwTouaiWibdQtCTg@mail.gmail.com>
Date: Mon, 14 Aug 2023 11:04:51 +0200
From: Geert Uytterhoeven <geert@...ux-m68k.org>
To: Rafał Miłecki <zajec5@...il.com>
Cc: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>, 
	Will Deacon <will@...nel.org>, Waiman Long <longman@...hat.com>, Boqun Feng <boqun.feng@...il.com>, 
	Russell King <linux@...linux.org.uk>, Daniel Lezcano <daniel.lezcano@...aro.org>, 
	Thomas Gleixner <tglx@...utronix.de>, Florian Fainelli <f.fainelli@...il.com>, linux-clk@...r.kernel.org, 
	"linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>, 
	Network Development <netdev@...r.kernel.org>, 
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, 
	OpenWrt Development List <openwrt-devel@...ts.openwrt.org>, 
	bcm-kernel-feedback-list <bcm-kernel-feedback-list@...adcom.com>
Subject: Re: ARM board lockups/hangs triggered by locks and mutexes

Hi Rafal,

On Mon, Aug 7, 2023 at 1:11 PM Rafał Miłecki <zajec5@...il.com> 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@...ux-m68k.org

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