linux-kernel - Re: [REPOST PATCH v6 0/4] kgdb: Fix kgdb_roundup

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Date:   Fri, 7 Dec 2018 10:40:24 -0800
From:   Doug Anderson <dianders@...omium.org>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     Jason Wessel <jason.wessel@...driver.com>,
        Daniel Thompson <daniel.thompson@...aro.org>, mhocko@...e.com,
        linux-sh@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>,
        kgdb-bugreport@...ts.sourceforge.net,
        Will Deacon <will.deacon@....com>,
        LKML <linux-kernel@...r.kernel.org>, dalias@...c.org,
        paulus@...ba.org, "H. Peter Anvin" <hpa@...or.com>,
        sparclinux@...r.kernel.org,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        linux-hexagon@...r.kernel.org, x86@...nel.org,
        Russell King - ARM Linux <linux@...linux.org.uk>,
        Ingo Molnar <mingo@...hat.com>, jhogan@...nel.org,
        linux-snps-arc@...ts.infradead.org, ying.huang@...el.com,
        linux-mips@...r.kernel.org, rppt@...ux.vnet.ibm.com, bp@...en8.de,
        Thomas Gleixner <tglx@...utronix.de>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        christophe.leroy@....fr, Vineet Gupta <vgupta@...opsys.com>,
        Ralf Baechle <ralf@...ux-mips.org>, rkuo@...eaurora.org,
        paul.burton@...s.com,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        mpe@...erman.id.au, Andrew Morton <akpm@...ux-foundation.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        David Miller <davem@...emloft.net>
Subject: Re: [REPOST PATCH v6 0/4] kgdb: Fix kgdb_roundup_cpus()

Hi,

On Fri, Dec 7, 2018 at 9:42 AM Catalin Marinas <catalin.marinas@....com> wrote:
>
> On Tue, Dec 04, 2018 at 07:38:24PM -0800, Douglas Anderson wrote:
> > Douglas Anderson (4):
> >   kgdb: Remove irq flags from roundup
> >   kgdb: Fix kgdb_roundup_cpus() for arches who used smp_call_function()
> >   kgdb: Don't round up a CPU that failed rounding up before
> >   kdb: Don't back trace on a cpu that didn't round up
>
> FWIW, trying these on arm64 (ThunderX2) with CONFIG_KGDB_TESTS_ON_BOOT=y
> on top of 4.20-rc5 doesn't boot. It looks like they leave interrupts
> disabled when they shouldn't and it trips over the BUG at
> mm/vmalloc.c:1380 (called via do_fork -> copy_process).
>
> Now, I don't think these patches make things worse on arm64 since prior
> to them the kgdb boot tests on arm64 were stuck in a loop (RUN
> singlestep).

Thanks for the report!  ...actually, I'd never tried CONFIG_KGDB_TESTS
before.  ...so I tried them now:

A) chromeos-4.19 tree on qcom-sdm845 without this series: booted up OK
B) chromeos-4.19 tree on qcom-sdm845 with this series: booted up OK
C) v4.20-rc5-90-g30002dd008ed on rockchip-rk3399 (kevin) with this
series: booted up OK

Example output from B) above:

localhost ~ # dmesg | grep kgdbts
[    2.139814] KGDB: Registered I/O driver kgdbts
[    2.144582] kgdbts:RUN plant and detach test
[    2.165333] kgdbts:RUN sw breakpoint test
[    2.172990] kgdbts:RUN bad memory access test
[    2.178640] kgdbts:RUN singlestep test 1000 iterations
[    2.187765] kgdbts:RUN singlestep [0/1000]
[    2.559596] kgdbts:RUN singlestep [100/1000]
[    2.931419] kgdbts:RUN singlestep [200/1000]
[    3.303474] kgdbts:RUN singlestep [300/1000]
[    3.675121] kgdbts:RUN singlestep [400/1000]
[    4.046867] kgdbts:RUN singlestep [500/1000]
[    4.418920] kgdbts:RUN singlestep [600/1000]
[    4.790824] kgdbts:RUN singlestep [700/1000]
[    5.162479] kgdbts:RUN singlestep [800/1000]
[    5.534103] kgdbts:RUN singlestep [900/1000]
[    5.902299] kgdbts:RUN do_fork for 100 breakpoints
[    8.463900] KGDB: Unregistered I/O driver kgdbts, debugger disabled

...so I guess I'm a little confused.  Either I have a different config
than you do or something is special about your machine?


NOTE: In general I've never considered "single step" as reliable in
kgdb.  I mostly use kgdb as "after the fact" crash debugging to
analyze local variables / memory / other tasks.  If it worked that'd
actually be kinda great, but at least when I started using kgdb years
ago I learned that it didn't work and stopped trying...


-Doug