linux-kernel - Re: 2.6.25-mm1: not looking good

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Date:	Fri, 18 Apr 2008 13:46:48 +0200
From:	"Vegard Nossum" <vegard.nossum@...il.com>
To:	"Ingo Molnar" <mingo@...e.hu>
Cc:	"Jason Wessel" <jason.wessel@...driver.com>,
	"Andrew Morton" <akpm@...ux-foundation.org>, tglx@...utronix.de,
	penberg@...helsinki.fi, linux-usb@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	jmorris@...ei.org, sds@...ho.nsa.gov
Subject: Re: 2.6.25-mm1: not looking good

On Fri, Apr 18, 2008 at 9:37 AM, Ingo Molnar <mingo@...e.hu> wrote:
>
>  * Jason Wessel <jason.wessel@...driver.com> wrote:
>
>  > > [...] The final initcall is init_kgdbts() and disabling KGDB
>  > > prevents the hang.
>
>
>  > That enables verbose logging of exactly what is going on and will show
>  > where wheels fall off the cart.  If the kernel is dying silently it
>  > means the early exception code has completely failed in some way on
>  > the kernel architecture that was selected, and of course the .config
>  > is always useful in this case.
>
>  incidentally, just today, in overnight testing i triggered a similar
>  hang in the KGDB self-test:
>
>   http://redhat.com/~mingo/misc/config-Thu_Apr_17_23_46_36_CEST_2008.bad
>
>  to get a similar tree to the one i tested, pick up sched-devel/latest
>  from:
>
>    http://people.redhat.com/mingo/sched-devel.git/README
>
>  pick up that failing .config, do 'make oldconfig' and accept all the
>  defaults to get a comparable kernel to mine. (kgdb is embedded in
>  sched-devel.git.)
>
>  the hang was at:
>
>  [   12.504057] Calling initcall 0xffffffff80b800c1: init_kgdbts+0x0/0x1b()
>  [   12.511298] kgdb: Registered I/O driver kgdbts.
>  [   12.515062] kgdbts:RUN plant and detach test
>  [   12.520283] kgdbts:RUN sw breakpoint test
>  [   12.524651] kgdbts:RUN bad memory access test
>  [   12.529052] kgdbts:RUN singlestep breakpoint test
>
>  full log:
>
>   http://redhat.com/~mingo/misc/log-Thu_Apr_17_23_46_36_CEST_2008.bad
>
>  note that this was a 64-bit config too - our tests do a perfect mix of
>  50% 32-bit and 50% 64-bit kernels. So single-stepping of the kernel
>  broke in some circumstances.
>
>  find the boot log below. (it also includes all command line parameters)
>
>  This is the first time ever i saw the self-test in KGDB hanging, so it's
>  some recent non-KGDB change that provoked it or made it more likely. The
>  KGDB self-test runs very frequently in my bootup tests:
>
>  [   12.508236] kgdb: Registered I/O driver kgdbts.
>  [   12.511245] kgdbts:RUN plant and detach test
>  [   12.517418] kgdbts:RUN sw breakpoint test
>  [   12.521056] kgdbts:RUN bad memory access test
>  [   12.525515] kgdbts:RUN singlestep breakpoint test
>  [   12.531483] kgdbts:RUN hw breakpoint test
>  [   12.536142] kgdbts:RUN hw write breakpoint test
>  [   12.541007] kgdbts:RUN access write breakpoint test
>  [   12.546223] kgdbts:RUN do_fork for 100 breakpoints
>
>  so the latest kgdb-light tree literally survived thousands of such tests
>  since it was changed last.
>
>  unfortunately, the condition was not reproducible - i booted it once
>  more and then it came up just fine - using the same bzImage.
>
>  there's no recent change in x86.git related to the TF flag that i could
>  think of to cause something like this. I checked changes to traps_64.c
>  and entry_64.S, and nothing suspicious.

With the patch below, it seems 100% reproducible to me (7 out of 7
bootups hung).

The number of loops it could do before hanging were, in order: 697,
898, 237, 55, 45, 92, 59

It seems timing-related, so I'm guessing it could be some interaction
with interrupts?


Vegard


diff --git a/drivers/misc/kgdbts.c b/drivers/misc/kgdbts.c
index 6d6286c..ee87820 100644
--- a/drivers/misc/kgdbts.c
+++ b/drivers/misc/kgdbts.c
@@ -895,7 +895,13 @@ static void kgdbts_run_tests(void)
        v1printk("kgdbts:RUN bad memory access test\n");
        run_bad_read_test();
        v1printk("kgdbts:RUN singlestep breakpoint test\n");
-       run_singlestep_break_test();
+
+       while(1) {
+               static int i = 0;
+
+               run_singlestep_break_test();
+               printk(KERN_EMERG "test #%d successfull\n", i++);
+       }

        /* ===Optional tests=== */
--
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