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From: Christophe Leroy <>
To: Rasmus Villemoes <>,
	Li Yang <>, Qiang Zhao <>,
	Greg Kroah-Hartman <>
Cc: "" <>,
	Scott Wood <>,,
	LKML <>
Subject: Re: [Regression 5.6-rc1][Bisected b6231ea2b3c6] Powerpc 8xx doesn't boot anymore
Date: Thu, 13 Feb 2020 10:40:21 +0000	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

On 02/13/2020 07:45 AM, Rasmus Villemoes wrote:
> On 12/02/2020 15.24, Christophe Leroy wrote:
>> Hi Rasmus,
>> Kernel 5.6-rc1 silently fails on boot.
>> I bisected the problem to commit b6231ea2b3c6 ("soc: fsl: qe: drop
>> broken lazy call of cpm_muram_init()")
>> I get a bad_page_fault() for an access at address 8 in
>> cpm_muram_alloc_common(), called from cpm_uart_console_setup() via
>> cpm_uart_allocbuf()
> Sorry about that. But I'm afraid I don't see what I could have done
> differently - the patch series, including b6231ea2b3c6, has been in
> -next since 20191210, both you and ppc-dev were cc'ed on the entire
> series (last revision sent November 28). And I've been dogfooding the
> patches on both arm- and ppc-derived boards ever since (but obviously
> only for a few cpus).

Yes, this patch series should have ringed a bell in my head, looks like 
I'm the one who introduced this 4 years ago through commit 4d486e008379 
("soc/fsl/qe: fix Oops on CPM1 (and likely CPM2)")

But I had completely forgotten that patch until I did some git blame 
this morning on this lazy call.

>> Reverting the guilty commit on top of 5.6-rc1 is not trivial.
>> In your commit text you explain that cpm_muram_init() is called via
>> subsys_initcall. But console init is done before that, so it cannot work.
> No, but neither did the code I removed seem to work - how does doing
> spin_lock_init on a held spinlock, and then unlocking it, work? Is
> everything-spinlock always a no-op in your configuration? And even so,
> I'd think a GFP_KERNEL allocation under spin_lock_irqsave() would
> trigger some splat somewhere?
> Please note I'm not claiming my patch is not at fault, it clearly is, I
> just want to try to understand how I could have been wrong about the
> "nobody can have been relying on it" part.

It seems spin_lock_init() does just nothing.
spin_lock_irqsave() just disable IRQs and increases preempt_count.
spin_lock_irqrestore() restore IRQ state, decreace preempt_count and 
call preempt_schedule if preempt_count reaches 0.

Maybe with some debugging options like DEBUG_ATOMIC_SLEEP could detect it ?

>> Do you have a fix for that ?
> Not right now, but I'll have a look. It's true that the patch probably
> doesn't revert cleanly, but it shouldn't be hard to add back those few
> lines in the appropriate spot, with a big fat comment that this does
> something very fishy (at least as a temporary measure if we don't find a
> proper solution soonish).


      reply	other threads:[~2020-02-13 10:40 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-02-12 14:24 Christophe Leroy
2020-02-12 14:42 ` Christophe Leroy
2020-02-12 14:50 ` Christophe Leroy
2020-02-13  3:35   ` Qiang Zhao
2020-02-13  6:17     ` Christophe Leroy
2020-02-13  6:36       ` Qiang Zhao
2020-02-13  9:37   ` Rasmus Villemoes
2020-02-13 10:50     ` Christophe Leroy
2020-02-13  7:45 ` Rasmus Villemoes
2020-02-13 10:40   ` Christophe Leroy [this message]

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