Messages in this thread | | | Subject | Re: [Regression 5.6-rc1][Bisected b6231ea2b3c6] Powerpc 8xx doesn't boot anymore | From | Christophe Leroy <> | Date | Thu, 13 Feb 2020 10:40:21 +0000 |
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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). >
Thanks Christophe
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